[clang] crash with "type should never be variably-modified"

Reproducer:
https://godbolt.org/z/4oKoezEG6
```cpp
struct S {
void foo(int n, auto... v[n] {};
};
```

Bacltrace:
```console
type should never be variably-modified
UNREACHABLE executed at /root/llvm-project/llvm/tools/clang/lib/AST/ASTContext.cpp:4346!
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. :2:34: current parser token ';'
2. :1:1: parsing struct/union/class body 'S'
#0 0x000000000435d468 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x435d468)
#1 0x000000000435a3a4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x435a3a4)
#2 0x000000000435a9c4 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x435a9c4)
#3 0x0000000004297e28 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x00007c5cd6445330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x00007c5cd649eb2c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb2c)
#6 0x00007c5cd644527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x00007c5cd64288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x00000000042a21ca (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42a21ca)
#9 0x0000000007c20bb2 clang::ASTContext::getVariableArrayDecayedType(clang::QualType) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7c20bb2)
#10 0x0000000007c24b76 clang::ASTContext::getCanonicalParamType(clang::QualType) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7c24b76)
#11 0x0000000007c2bea6 clang::ASTContext::getFunctionTypeInternal(clang::QualType, llvm::ArrayRef, clang::FunctionProtoType::ExtProtoInfo const&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7c2bea6)
#12 0x00000000078e49df GetFullTypeForDeclarator((anonymous namespace)::TypeProcessingState&, clang::QualType, clang::TypeSourceInfo*) SemaType.cpp:0:0
#13 0x00000000078e9124 clang::Sema::GetTypeForDeclarator(clang::Declarator&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x78e9124)
#14 0x000000000705d654 clang::Sema::HandleDeclarator(clang::Scope*, clang::Declarator&, llvm::MutableArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x705d654)
#15 0x0000000007104e1f clang::Sema::ActOnCXXMemberDeclarator(clang::Scope*, clang::AccessSpecifier, clang::Declarator&, llvm::MutableArrayRef, clang::Expr*, clang::VirtSpecifiers const&, clang::InClassInitStyle) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7104e1f)
#16 0x0000000006cc33be clang::Parser::ParseCXXClassMemberDeclaration(clang::AccessSpecifier, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cc33be)
#17 0x0000000006cc4aff clang::Parser::ParseCXXClassMemberDeclarationWithPragmas(clang::AccessSpecifier&, clang::ParsedAttributes&, clang::TypeSpecifierType, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cc4aff)
#18 0x0000000006cc67df clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6cc67df)
#19 0x0000000006cc917b 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+++0x6cc917b)
#20 0x0000000006c9cacc 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+++0x6c9cacc)
#21 0x0000000006c57527 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c57527)
#22 0x0000000006c57faf clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c57faf)
#23 0x0000000006c5fa61 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c5fa61)
#24 0x0000000006c60975 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c60975)
#25 0x0000000006c60d60 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c60d60)
#26 0x0000000006c41823 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6c41823)
#27 0x0000000004e583d8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e583d8)
#28 0x0000000005193a38 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5193a38)
#29 0x000000000510f0fd clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x510f0fd)
#30 0x0000000005292a3d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5292a3d)
#31 0x0000000000dccf5c cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdccf5c)
#32 0x0000000000dc3c3a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#33 0x0000000000dc3dbd int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#34 0x0000000004ecbbc9 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#35 0x0000000004298264 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4298264)
#36 0x0000000004ecc216 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#37 0x0000000004e883c2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e883c2)
#38 0x0000000004e8936e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e8936e)
#39 0x0000000004e90ab5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e90ab5)
#40 0x0000000000dc94d2 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdc94d2)
#41 0x0000000000cc900a main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcc900a)
#42 0x00007c5cd642a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#43 0x00007c5cd642a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#44 0x0000000000dc35e5 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdc35e5)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 134
```

Start by reproducing the Godbolt example, then inspect clang::ASTContext::getVariableArrayDecayedType in llvm/tools/clang/lib/AST/ASTContext.cpp and the declarator handling in SemaType.cpp. The fix should prevent this valid-looking input from reaching the unreachable assertion and include regression coverage for the reproducer.

Written by the indexing model from the issue text.