[Clang] Crash on fold expression over structured binding pack in expansion statement

This crashes in CodeGen (https://godbolt.org/z/zs6ze3Kr3):
```c++
struct S {
int x;
int y;
};

int f() {
S s{1, 2};
template for (int _ : {0}) {
auto [...members] = s;
return (members + ...);
}
}
```
Attempting to constant-evaluate `f()` crashes in the constant evaluator instead (https://godbolt.org/z/3az5b6sGG).

### CodeGen Crash
Assertion:
```console
Unexpected placeholder builtin type!
UNREACHABLE executed at /root/llvm-project/llvm/tools/clang/lib/CodeGen/CodeGenTypes.cpp:602!
```
Stack Trace:
```console
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++26
1. parser at end of file
2. :6:5: LLVM IR generation of declaration 'f'
3. :6:5: Generating code for declaration 'f'
4. :8:32: LLVM IR generation of compound statement ('{}')
5. :8:32: LLVM IR generation of compound statement ('{}')
#0 0x000000000448cc18 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x448cc18)
#1 0x0000000004489e74 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4489e74)
#2 0x000000000448a4ac llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x448a4ac)
#3 0x00000000043c71c8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x00007b233ae45330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x00007b233ae9ec0c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9ec0c)
#6 0x00007b233ae4527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x00007b233ae288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x00000000043d14ca (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43d14ca)
#9 0x0000000004c6df56 clang::CodeGen::CodeGenTypes::ConvertType(clang::QualType) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c6df56)
#10 0x00000000049d960c (anonymous namespace)::ScalarExprEmitter::VisitExpr(clang::Expr*) (.isra.0) CGExprScalar.cpp:0:0
#11 0x00000000049ef778 clang::StmtVisitorBase::Visit(clang::Stmt*) CGExprScalar.cpp:0:0
#12 0x00000000049f519c clang::CodeGen::CodeGenFunction::EmitScalarExpr(clang::Expr const*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49f519c)
#13 0x0000000004b1a09d clang::CodeGen::CodeGenFunction::EmitReturnStmt(clang::ReturnStmt const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b1a09d)
#14 0x0000000004b2a12b clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2a12b)
#15 0x0000000004b2a44d clang::CodeGen::CodeGenFunction::EmitCompoundStmt(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2a44d)
#16 0x0000000004b2a5db clang::CodeGen::CodeGenFunction::EmitSimpleStmt(clang::Stmt const*, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2a5db)
#17 0x0000000004b21d99 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b21d99)
#18 0x0000000004b2a12b clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2a12b)
#19 0x0000000004b2a44d clang::CodeGen::CodeGenFunction::EmitCompoundStmt(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2a44d)
#20 0x0000000004b2a5db clang::CodeGen::CodeGenFunction::EmitSimpleStmt(clang::Stmt const*, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2a5db)
#21 0x0000000004b21d99 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b21d99)
#22 0x0000000004b257ad clang::CodeGen::CodeGenFunction::EmitCXXExpansionStmtInstantiation(clang::CXXExpansionStmtInstantiation const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b257ad)
#23 0x0000000004b107ec clang::CodeGen::CodeGenFunction::EmitDeclStmt(clang::DeclStmt const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b107ec)
#24 0x0000000004b2a577 clang::CodeGen::CodeGenFunction::EmitSimpleStmt(clang::Stmt const*, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2a577)
#25 0x0000000004b21d99 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b21d99)
#26 0x0000000004b2a12b clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2a12b)
#27 0x0000000004b93646 clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b93646)
#28 0x0000000004ba7e84 clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ba7e84)
#29 0x0000000004c19752 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c19752)
#30 0x0000000004c128c4 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c128c4)
#31 0x0000000004c13890 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c13890)
#32 0x0000000004c20839 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#33 0x0000000004fa0569 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#34 0x0000000004f91544 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f91544)
#35 0x0000000006ddcb84 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ddcb84)
#36 0x0000000004f9e238 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f9e238)
#37 0x00000000052ca888 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52ca888)
#38 0x000000000525839d clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x525839d)
#39 0x000000000539f0ed clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x539f0ed)
#40 0x0000000000d8dbac cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd8dbac)
#41 0x0000000000d848aa ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#42 0x0000000000d84a2d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#43 0x0000000005012199 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#44 0x00000000043c75c4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43c75c4)
#45 0x0000000005012d05 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#46 0x0000000004fd08e2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fd08e2)
#47 0x0000000004fd5247 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fd5247)
#48 0x0000000004fdf6d5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fdf6d5)
#49 0x0000000000d8a09a clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd8a09a)
#50 0x0000000000cc7dfa main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcc7dfa)
```

### Constant Evaluator Crash
Assertion:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/AST/ExprConstant.cpp:5728:
bool EvaluateDependentExpr(const clang::Expr*, {anonymous}::EvalInfo&):
Assertion `E->containsErrors() && "valid value-dependent expression should never " "reach invalid code path."' failed.
```

Stack trace:
```console
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -std=c++26
1. parser at end of file
#0 0x000000000448cc18 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x448cc18)
#1 0x0000000004489e74 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4489e74)
#2 0x000000000448a4ac llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x448a4ac)
#3 0x00000000043c71c8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x00007a0865445330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x00007a086549ec0c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9ec0c)
#6 0x00007a086544527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x00007a08654288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x00007a086542881b (/lib/x86_64-linux-gnu/libc.so.6+0x2881b)
#9 0x00007a086543b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#10 0x00000000081a7991 EvaluateDependentExpr(clang::Expr const*, (anonymous namespace)::EvalInfo&) ExprConstant.cpp:0:0
#11 0x000000000822ece6 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#12 0x000000000822dd38 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#13 0x000000000822dd38 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#14 0x000000000822de11 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#15 0x000000000822d234 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#16 0x000000000822dd38 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#17 0x000000000823578b HandleFunctionCall(clang::SourceLocation, clang::FunctionDecl const*, (anonymous namespace)::LValue const*, clang::Expr const*, llvm::ArrayRef, (anonymous namespace)::CallRef, clang::Stmt const*, (anonymous namespace)::EvalInfo&, clang::APValue&, (anonymous namespace)::LValue const*) ExprConstant.cpp:0:0
#18 0x0000000008264390 (anonymous namespace)::IntExprEvaluator::VisitCallExpr(clang::CallExpr const*) ExprConstant.cpp:0:0
#19 0x00000000081ffcb2 clang::StmtVisitorBase::Visit(clang::Stmt const*) ExprConstant.cpp:0:0
#20 0x00000000081e88df Evaluate(clang::APValue&, (anonymous namespace)::EvalInfo&, clang::Expr const*) ExprConstant.cpp:0:0
#21 0x00000000081e9692 EvaluateAsRValue((anonymous namespace)::EvalInfo&, clang::Expr const*, clang::APValue&) ExprConstant.cpp:0:0
#22 0x00000000081ede9a clang::Expr::isCXX11ConstantExpr(clang::ASTContext const&, clang::APValue*, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x81ede9a)
#23 0x00000000081f1d5c clang::Expr::getIntegerConstantExpr(clang::ASTContext const&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x81f1d5c)
#24 0x000000000709fed0 AnalyzeImplicitConversions(clang::Sema&, clang::Expr*, clang::SourceLocation, bool) (.constprop.0) SemaChecking.cpp:0:0
#25 0x00000000070a0d75 clang::Sema::CheckCompletedExpr(clang::Expr*, clang::SourceLocation, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x70a0d75)
#26 0x0000000007470e87 clang::Sema::ActOnFinishFullExpr(clang::Expr*, clang::SourceLocation, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7470e87)
#27 0x00000000072aa215 clang::Sema::BuildStaticAssertDeclaration(clang::SourceLocation, clang::Expr*, clang::Expr*, clang::SourceLocation, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x72aa215)
#28 0x0000000006e51da0 clang::Parser::ParseStaticAssertDeclaration(clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e51da0)
#29 0x0000000006e48fd6 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e48fd6)
#30 0x0000000006dff8b0 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dff8b0)
#31 0x0000000006e007c5 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e007c5)
#32 0x0000000006ddcb9a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ddcb9a)
#33 0x0000000004f9e238 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f9e238)
#34 0x00000000052ca888 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52ca888)
#35 0x000000000525839d clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x525839d)
#36 0x000000000539f0ed clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x539f0ed)
#37 0x0000000000d8dbac cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd8dbac)
#38 0x0000000000d848aa ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#39 0x0000000000d84a2d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#40 0x0000000005012199 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#41 0x00000000043c75c4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43c75c4)
#42 0x0000000005012d05 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#43 0x0000000004fd08e2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fd08e2)
#44 0x0000000004fd5247 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fd5247)
#45 0x0000000004fdf6d5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fdf6d5)
#46 0x0000000000d8a09a clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd8a09a)
#47 0x0000000000cc7dfa main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcc7dfa)
```

Start by compiling the structured-binding-pack reproducer with C++26 and inspect the CodeGen and constant-evaluation stack traces. Read CodeGenTypes.cpp, CGExprScalar.cpp, and ExprConstant.cpp around the reported assertions, then find the relevant Clang regression-test locations. Done means both CodeGen and constant evaluation handle the example without crashing.

Written by the indexing model from the issue text.