Using ?: operator in fold expression -> Assertion `!(D & ExprDependence::UnexpandedPack)' failed.

[Godbolt link](https://godbolt.org/z/7KMbKGfrG)
```cpp
template
void foo(A ...a) {
(a ?: 42, ...);
}
```

```
clang++: /root/llvm-project/llvm/tools/clang/lib/AST/ComputeDependence.cpp:32: clang::ExprDependence clang::computeDependence(clang::OpaqueValueExpr*): Assertion `!(D & ExprDependence::UnexpandedPack)' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
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
1. :3:10: current parser token ','
2. :2:18: parsing function body 'foo'
3. :2:18: in compound statement ('{}')
#0 0x00000000041ca628 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41ca628)
#1 0x00000000041c7a54 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41c7a54)
#2 0x000000000410bce8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x00007c47ee842520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x00007c47ee8969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x00007c47ee842476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x00007c47ee8287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x00007c47ee82871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#8 0x00007c47ee839e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#9 0x00000000079689fa clang::computeDependence(clang::OpaqueValueExpr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x79689fa)
#10 0x0000000006e23014 clang::Sema::ActOnConditionalOp(clang::SourceLocation, clang::SourceLocation, clang::Expr*, clang::Expr*, clang::Expr*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e23014)
#11 0x000000000692b2cf clang::Parser::ParseRHSOfBinaryExpression(clang::ActionResult, clang::prec::Level) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x692b2cf)
#12 0x000000000692e169 clang::Parser::ParseExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x692e169)
#13 0x0000000006931f04 clang::Parser::ParseParenExpression(clang::ParenParseOption&, bool, clang::ParenExprKind, clang::TypoCorrectionTypeBehavior, clang::OpaquePtr&, clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6931f04)
#14 0x0000000006928538 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, bool&, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6928538)
#15 0x00000000069298a7 clang::Parser::ParseCastExpression(clang::CastParseKind, bool, clang::TypoCorrectionTypeBehavior, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69298a7)
#16 0x0000000006929939 clang::Parser::ParseAssignmentExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6929939)
#17 0x000000000692e169 clang::Parser::ParseExpression(clang::TypoCorrectionTypeBehavior) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x692e169)
#18 0x00000000069b2d89 clang::Parser::ParseExprStatement(clang::Parser::ParsedStmtContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69b2d89)
#19 0x00000000069aa112 clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69aa112)
#20 0x00000000069aaf89 clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::LabelDecl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69aaf89)
#21 0x00000000069b349e clang::Parser::ParseCompoundStatementBody(bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69b349e)
#22 0x00000000069b3cba clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69b3cba)
#23 0x00000000068ba1bb clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68ba1bb)
#24 0x00000000068f062d clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68f062d)
#25 0x00000000069bf9b8 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69bf9b8)
#26 0x00000000069ccb99 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#27 0x00000000069ccfaa clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x69ccfaa)
#28 0x00000000068f8f30 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68f8f30)
#29 0x00000000068b5627 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68b5627)
#30 0x00000000068b64f5 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68b64f5)
#31 0x00000000068b69b0 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68b69b0)
#32 0x0000000006897b23 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6897b23)
#33 0x0000000004b08ee8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b08ee8)
#34 0x0000000004df44e5 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4df44e5)
#35 0x0000000004d754be clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4d754be)
#36 0x0000000004eeb75d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4eeb75d)
#37 0x0000000000dbbd40 cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdbbd40)
#38 0x0000000000db28aa ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#39 0x0000000000db2a2d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#40 0x0000000004b72069 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#41 0x000000000410c184 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x410c184)
#42 0x0000000004b7267f clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#43 0x0000000004b339b2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b339b2)
#44 0x0000000004b3495e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b3495e)
#45 0x0000000004b3c095 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b3c095)
#46 0x0000000000db8269 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdb8269)
#47 0x0000000000c69a74 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc69a74)
#48 0x00007c47ee829d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#49 0x00007c47ee829e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#50 0x0000000000db2345 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdb2345)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
```

Reproduce the assertion with the C++ template from the Godbolt link using an assertions-enabled Clang build. Start in llvm/tools/clang/lib/AST/ComputeDependence.cpp and follow the Sema::ActOnConditionalOp entry point shown in the stack trace. Done means the reproducer no longer aborts with the UnexpandedPack assertion.

Written by the indexing model from the issue text.