[clang] UNREACHABLE executed in SemaTemplateDeduction.cpp

Reproducer:
https://godbolt.org/z/6cM41P5EE
```cpp
template class C> struct S {
static foo bar;
template class C2> using baz = C2;
using C2 = baz;
};
```

Backtrace:
```console
Unexpected Result
UNREACHABLE executed at /root/llvm-project/llvm/tools/clang/lib/Sema/SemaTemplateDeduction.cpp:6644!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and dumped files.
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. :4:14: at annotation token
2. :1:67: parsing struct/union/class body 'S'
#0 0x0000000004422c18 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4422c18)
#1 0x000000000441fac4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x441fac4)
#2 0x00000000044200fc llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44200fc)
#3 0x000000000435f0b8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x000078d761645330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x000078d76169eb2c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb2c)
#6 0x000078d76164527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x000078d7616288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x000000000436942a (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x436942a)
#9 0x0000000007819082 clang::Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(clang::TemplateParameterList*, clang::TemplateDecl*, clang::TemplateDecl*, clang::DefaultArguments const&, clang::SourceLocation, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7819082)
#10 0x000000000778e7e5 clang::Sema::CheckTemplateTemplateArgument(clang::TemplateTemplateParmDecl*, clang::TemplateParameterList*, clang::TemplateArgumentLoc&, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x778e7e5)
#11 0x0000000007799995 clang::Sema::CheckTemplateArgument(clang::NamedDecl*, clang::TemplateArgumentLoc&, clang::NamedDecl*, clang::SourceLocation, clang::SourceLocation, unsigned int, clang::Sema::CheckTemplateArgumentInfo&, clang::Sema::CheckTemplateArgumentKind) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7799995)
#12 0x000000000779e9e5 clang::Sema::CheckTemplateArgumentList(clang::TemplateDecl*, clang::TemplateParameterList*, clang::SourceLocation, clang::TemplateArgumentListInfo&, clang::DefaultArguments const&, bool, clang::Sema::CheckTemplateArgumentInfo&, bool, bool*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x779e9e5)
#13 0x00000000077b44f7 clang::Sema::CheckTemplateIdType(clang::ElaboratedTypeKeyword, clang::TemplateName, clang::SourceLocation, clang::TemplateArgumentListInfo&, clang::Scope*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x77b44f7)
#14 0x00000000077b75a1 clang::Sema::ActOnTemplateIdType(clang::Scope*, clang::ElaboratedTypeKeyword, clang::SourceLocation, clang::CXXScopeSpec&, clang::SourceLocation, clang::OpaquePtr, clang::IdentifierInfo const*, clang::SourceLocation, clang::SourceLocation, llvm::MutableArrayRef, clang::SourceLocation, bool, bool, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x77b75a1)
#15 0x0000000006e548e2 clang::Parser::AnnotateTemplateIdTokenAsType(clang::CXXScopeSpec&, clang::ImplicitTypenameContext, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e548e2)
#16 0x0000000006d7fe5e clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d7fe5e)
#17 0x0000000006d82675 clang::Parser::ParseSpecifierQualifierList(clang::DeclSpec&, clang::ImplicitTypenameContext, clang::AccessSpecifier, clang::Parser::DeclSpecContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d82675)
#18 0x0000000006d82b75 clang::Parser::ParseTypeName(clang::SourceRange*, clang::DeclaratorContext, clang::AccessSpecifier, clang::Decl**, clang::ParsedAttributes*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d82b75)
#19 0x0000000006d9b144 clang::Parser::ParseAliasDeclarationAfterDeclarator(clang::Parser::ParsedTemplateInfo const&, clang::SourceLocation, clang::Parser::UsingDeclarator&, clang::SourceLocation&, clang::AccessSpecifier, clang::ParsedAttributes&, clang::Decl**) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d9b144)
#20 0x0000000006da927a clang::Parser::ParseUsingDeclaration(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo const&, clang::SourceLocation, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6da927a)
#21 0x0000000006dab1c8 clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dab1c8)
#22 0x0000000006dacf9f clang::Parser::ParseCXXClassMemberDeclarationWithPragmas(clang::AccessSpecifier&, clang::ParsedAttributes&, clang::TypeSpecifierType, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dacf9f)
#23 0x0000000006dad51f clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dad51f)
#24 0x0000000006db008b clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6db008b)
#25 0x0000000006d7f401 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::LateParsedAttrList*, clang::ImplicitTypenameContext) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d7f401)
#26 0x0000000006e55ef9 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e55ef9)
#27 0x0000000006e625c3 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#28 0x0000000006e629ba clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e629ba)
#29 0x0000000006d92fb0 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d92fb0)
#30 0x0000000006d49820 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d49820)
#31 0x0000000006d4a755 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d4a755)
#32 0x0000000006d4ab40 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d4ab40)
#33 0x0000000006d27c4e clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d27c4e)
#34 0x0000000004f22a58 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f22a58)
#35 0x000000000524fc18 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x524fc18)
#36 0x00000000051dc79d clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x51dc79d)
#37 0x000000000532462d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x532462d)
#38 0x0000000000dab7ac cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdab7ac)
#39 0x0000000000da23ca ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#40 0x0000000000da254d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#41 0x0000000004f9ab99 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#42 0x000000000435f4f4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x435f4f4)
#43 0x0000000004f9b705 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#44 0x0000000004f53412 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f53412)
#45 0x0000000004f57d77 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f57d77)
#46 0x0000000004f62435 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f62435)
#47 0x0000000000da7caa clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda7caa)
#48 0x0000000000cc39fa main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcc39fa)
#49 0x000078d76162a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#50 0x000078d76162a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#51 0x0000000000da1d75 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda1d75)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 134
```

Reproduce the crash with the linked Compiler Explorer example, then start at isTemplateTemplateParameterAtLeastAsSpecializedAs in clang/lib/Sema/SemaTemplateDeduction.cpp around line 6644. Trace the template-template argument path shown in the backtrace and add a regression test for the reproducer; done means Clang reports a diagnostic or handles the input without executing UNREACHABLE.

Written by the indexing model from the issue text.