[Clang] Crash on fold expression involving lambda with requires clause

```c++
template
constexpr auto count_invocable{[] (auto... args) {
return (0U + ... +
requires { [] (auto) requires (sizeof(args) > 1) {}(0); }
);
}};

static_assert(count_invocable(0, 1) == 2);
```

See https://compiler-explorer.com/z/3nTnWvadd.

Assertion:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/AST/ExprCXX.cpp:2038:
clang::CXXFoldExpr::CXXFoldExpr(clang::QualType, clang::UnresolvedLookupExpr*, clang::SourceLocation, clang::Expr*, clang::BinaryOperatorKind, clang::SourceLocation, clang::Expr*, clang::SourceLocation, clang::UnsignedOrNone):
Assertion `LHS->containsUnexpandedParameterPack() != RHS->containsUnexpandedParameterPack() && "Exactly one of LHS or RHS should contain an unexpanded pack"' failed.
```

Stack dump

```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++20
1. :8:36: current parser token '('
#0 0x000000000458fa58 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x458fa58)
#1 0x000000000458c894 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x458c894)
#2 0x000000000458ceb4 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x458ceb4)
#3 0x00000000044c9428 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x000076c5b8c42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#5 0x000076c5b8c969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#6 0x000076c5b8c42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#7 0x000076c5b8c287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#8 0x000076c5b8c2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#9 0x000076c5b8c39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#10 0x000000000839722c (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x839722c)
#11 0x0000000007bb0cee clang::Sema::BuildCXXFoldExpr(clang::UnresolvedLookupExpr*, clang::SourceLocation, clang::Expr*, clang::BinaryOperatorKind, clang::SourceLocation, clang::Expr*, clang::SourceLocation, clang::OptionalUnsigned) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7bb0cee)
#12 0x0000000007ada2bf clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCXXFoldExpr(clang::CXXFoldExpr*) SemaTemplateInstantiate.cpp:0:0
#13 0x0000000007ab2eb8 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#14 0x0000000007ad3a18 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformInitializer(clang::Expr*, bool) (.part.0) SemaTemplateInstantiate.cpp:0:0
#15 0x0000000007ad3ddc clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformReturnStmt(clang::ReturnStmt*) SemaTemplateInstantiate.cpp:0:0
#16 0x0000000007af1c35 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCompoundStmt(clang::CompoundStmt*, bool) SemaTemplateInstantiate.cpp:0:0
#17 0x0000000007ab27f5 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformLambdaExpr(clang::LambdaExpr*) SemaTemplateInstantiate.cpp:0:0
#18 0x0000000007ab36c1 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#19 0x0000000007ad367b clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExprs(clang::Expr* const*, unsigned int, bool, llvm::SmallVectorImpl&, bool*) SemaTemplateInstantiate.cpp:0:0
#20 0x0000000007ad6e6c clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformInitListExpr(clang::InitListExpr*) SemaTemplateInstantiate.cpp:0:0
#21 0x0000000007ab3121 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformExpr(clang::Expr*) SemaTemplateInstantiate.cpp:0:0
#22 0x0000000007ad3a18 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformInitializer(clang::Expr*, bool) (.part.0) SemaTemplateInstantiate.cpp:0:0
#23 0x0000000007ad4674 clang::Sema::SubstInitializer(clang::Expr*, clang::MultiLevelTemplateArgumentList const&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7ad4674)
#24 0x0000000007b429e8 clang::Sema::InstantiateVariableInitializer(clang::VarDecl*, clang::VarDecl*, clang::MultiLevelTemplateArgumentList const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7b429e8)
#25 0x0000000007b9640c clang::Sema::BuildVariableInstantiation(clang::VarDecl*, clang::VarDecl*, clang::MultiLevelTemplateArgumentList const&, llvm::SmallVector*, clang::DeclContext*, clang::LocalInstantiationScope*, bool, clang::VarTemplateSpecializationDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7b9640c)
#26 0x0000000007b96a11 clang::TemplateDeclInstantiator::VisitVarTemplateSpecializationDecl(clang::VarTemplateDecl*, clang::VarDecl*, llvm::ArrayRef, clang::VarTemplateSpecializationDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7b96a11)
#27 0x0000000007ba2b1a clang::Sema::BuildVarTemplateInstantiation(clang::VarTemplateDecl*, clang::VarDecl*, clang::TemplateArgumentList const*, llvm::SmallVectorImpl&, clang::SourceLocation, llvm::SmallVector*, clang::LocalInstantiationScope*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7ba2b1a)
#28 0x000000000798d09a clang::Sema::CheckVarTemplateId(clang::VarTemplateDecl*, clang::SourceLocation, clang::SourceLocation, clang::TemplateArgumentListInfo const&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x798d09a)
#29 0x000000000798dba4 clang::Sema::CheckVarTemplateId(clang::CXXScopeSpec const&, clang::DeclarationNameInfo const&, clang::VarTemplateDecl*, clang::NamedDecl*, clang::SourceLocation, clang::TemplateArgumentListInfo const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x798dba4)
#30 0x000000000798de07 clang::Sema::BuildTemplateIdExpr(clang::CXXScopeSpec const&, clang::SourceLocation, clang::LookupResult&, bool, clang::TemplateArgumentListInfo const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x798de07)
#31 0x00000000074ac328 clang::Sema::ActOnIdExpression(clang::Scope*, clang::CXXScopeSpec&, clang::SourceLocation, clang::UnqualifiedId&, bool, bool, clang::CorrectionCandidateCallback*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x74ac328)
#32 0x0000000006fdda35 clang::Parser::tryParseCXXIdExpression(clang::CXXScopeSpec&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fdda35)
#33 0x0000000006fddd5e clang::Parser::ParseCXXIdExpression(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fddd5e)
#34 0x0000000006fc1f14 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc1f14)
#35 0x0000000006fc17f0 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fc17f0)
#36 0x0000000006fce79e clang::Parser::ParseConstantExpressionInExprEvalContext(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fce79e)
#37 0x0000000006fa5c16 clang::Parser::ParseStaticAssertDeclaration(clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6fa5c16)
#38 0x0000000006f944d6 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f944d6)
#39 0x0000000006f4ff56 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f4ff56)
#40 0x0000000006f50e65 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f50e65)
#41 0x0000000006f31d0a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6f31d0a)
#42 0x00000000050bbc58 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x50bbc58)
#43 0x00000000053ede05 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x53ede05)
#44 0x000000000536c868 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x536c868)
#45 0x00000000054ea6cd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x54ea6cd)
#46 0x0000000000e0a61b cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xe0a61b)
#47 0x0000000000e00f7a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#48 0x0000000000e010fd int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#49 0x000000000512e039 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#50 0x00000000044c9864 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44c9864)
#51 0x000000000512e686 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#52 0x00000000050ec722 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x50ec722)
#53 0x00000000050ed6ce clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x50ed6ce)
#54 0x00000000050f5385 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x50f5385)
#55 0x0000000000e069a4 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xe069a4)
#56 0x0000000000cae6ea main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcae6ea)
#57 0x000076c5b8c29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#58 0x000076c5b8c29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#59 0x0000000000e00a15 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xe00a15)
```

Reproduce the C++20 example with Clang assertions enabled and inspect clang/lib/AST/ExprCXX.cpp around the CXXFoldExpr assertion, then follow the instantiation path through SemaTemplateInstantiate.cpp. Determine why the lambda requires-clause produces invalid unexpanded-pack state; done means the example no longer crashes and regression coverage is added.

Written by the indexing model from the issue text.