[Clang] Assertion failure `Ctx->isFileContext() && "We should have been looking only at file context here already."` while parsing invalid template parameter with anonymous union and alignas

Code:
```CPP
template isFileContext() && "We should have been looking only at file context here already."' failed.
```

Backtrace:
```text
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. :1:61: current parser token 'b'
#0 0x00000000044e9f88 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44e9f88)
#1 0x00000000044e6db4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44e6db4)
#2 0x00000000044e73d4 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44e73d4)
#3 0x0000000004426678 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x00007eb034e42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#5 0x00007eb034e969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#6 0x00007eb034e42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#7 0x00007eb034e287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#8 0x00007eb034e2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#9 0x00007eb034e39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#10 0x00000000074b66f6 clang::Sema::CppLookupName(clang::LookupResult&, clang::Scope*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x74b66f6)
#11 0x00000000074b71ed clang::Sema::LookupName(clang::LookupResult&, clang::Scope*, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x74b71ed)
#12 0x00000000071b4e1d clang::Sema::getTypeName(clang::IdentifierInfo const&, clang::SourceLocation, clang::Scope*, clang::CXXScopeSpec*, bool, bool, clang::OpaquePtr, bool, bool, bool, clang::ImplicitTypenameContext, clang::IdentifierInfo**) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x71b4e1d)
#13 0x0000000006df1b37 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::Parser::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6df1b37)
#14 0x0000000006df6eea clang::Parser::ParseParameterDeclarationClause(clang::DeclaratorContext, clang::ParsedAttributes&, llvm::SmallVectorImpl&, clang::SourceLocation&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6df6eea)
#15 0x0000000006df829a clang::Parser::ParseFunctionDeclarator(clang::Declarator&, clang::ParsedAttributes&, clang::BalancedDelimiterTracker&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6df829a)
#16 0x0000000006dfd113 clang::Parser::ParseDirectDeclarator(clang::Declarator&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dfd113)
#17 0x0000000006dea142 clang::Parser::ParseDeclaratorInternal(clang::Declarator&, void (clang::Parser::*)(clang::Declarator&)) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dea142)
#18 0x00000000088438e1 clang::StackExhaustionHandler::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x88438e1)
#19 0x0000000006df6f61 clang::Parser::ParseParameterDeclarationClause(clang::DeclaratorContext, clang::ParsedAttributes&, llvm::SmallVectorImpl&, clang::SourceLocation&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6df6f61)
#20 0x0000000006df829a clang::Parser::ParseFunctionDeclarator(clang::Declarator&, clang::ParsedAttributes&, clang::BalancedDelimiterTracker&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6df829a)
#21 0x0000000006dfd113 clang::Parser::ParseDirectDeclarator(clang::Declarator&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dfd113)
#22 0x0000000006dea1bf clang::Parser::ParseDeclaratorInternal(clang::Declarator&, void (clang::Parser::*)(clang::Declarator&)) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dea1bf)
#23 0x00000000088438e1 clang::StackExhaustionHandler::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x88438e1)
#24 0x0000000006dd874f clang::Parser::ParseDeclarator(clang::Declarator&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dd874f)
#25 0x0000000006ec54c1 clang::Parser::ParseNonTypeTemplateParameter(unsigned int, unsigned int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ec54c1)
#26 0x0000000006ec715b clang::Parser::ParseTemplateParameter(unsigned int, unsigned int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ec715b)
#27 0x0000000006ec7d2e clang::Parser::ParseTemplateParameterList(unsigned int, llvm::SmallVectorImpl&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ec7d2e)
#28 0x0000000006ec850d clang::Parser::ParseTemplateParameters(clang::Parser::MultiParseScope&, unsigned int, llvm::SmallVectorImpl&, clang::SourceLocation&, clang::SourceLocation&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ec850d)
#29 0x0000000006ecae73 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#30 0x0000000006ecb3ea clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ecb3ea)
#31 0x0000000006df6b10 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6df6b10)
#32 0x0000000006db2f36 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db2f36)
#33 0x0000000006db3e25 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db3e25)
#34 0x0000000006db4210 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db4210)
#35 0x0000000006d94da3 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d94da3)
#36 0x0000000004fede98 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fede98)
#37 0x0000000005307a55 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5307a55)
#38 0x00000000052875ee clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52875ee)
#39 0x000000000540476d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x540476d)
#40 0x0000000000dda7f2 cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdda7f2)
#41 0x0000000000dd112a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#42 0x0000000000dd12ad int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#43 0x00000000050576f9 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#44 0x0000000004426ab4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4426ab4)
#45 0x0000000005057d26 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#46 0x0000000005016a72 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5016a72)
#47 0x0000000005017a1e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5017a1e)
#48 0x000000000501f6e5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x501f6e5)
#49 0x0000000000dd6b54 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdd6b54)
#50 0x0000000000c8e05a main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc8e05a)
#51 0x00007eb034e29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#52 0x00007eb034e29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#53 0x0000000000dd0bc5 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdd0bc5)
```

Goes back to Clang-16

See Compiler Explorer: [https://godbolt.org/z/hxMj33nab](https://godbolt.org/z/hxMj33nab)

The test case was generated by a fuzzer.

Reproduce the failure by running clang++ on the provided template input, then inspect llvm/tools/clang/lib/Sema/SemaLookup.cpp at the CppLookupName assertion. Follow the parser and lookup frames in the supplied backtrace to understand the invalid-template path. Done means the input produces diagnostics or recovery without triggering the assertion.

Written by the indexing model from the issue text.