[clang] Crash when using __declspec(selectany) on a struct with -fms-extensions

Reproducer:
https://godbolt.org/z/EM8Y4draa
```cpp
struct S {};
struct __declspec(selectany) S s;
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/include/llvm/Support/Casting.h:572: decltype(auto) llvm::cast(From*) [with To = clang::VarDecl; From = clang::Decl]: Assertion `isa(Val) && "cast() argument of incompatible type!"' 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 -fms-extensions
1. :2:32: current parser token 's'
#0 0x00000000044e6fc8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44e6fc8)
#1 0x00000000044e3df4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44e3df4)
#2 0x00000000044e4414 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44e4414)
#3 0x00000000044236b8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x000076211a242520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#5 0x000076211a2969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#6 0x000076211a242476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#7 0x000076211a2287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#8 0x000076211a22871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#9 0x000076211a239e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#10 0x0000000007176b39 clang::Sema::mergeDeclAttributes(clang::NamedDecl*, clang::Decl*, clang::AvailabilityMergeKind) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7176b39)
#11 0x0000000007178280 clang::Sema::ActOnTag(clang::Scope*, unsigned int, clang::TagUseKind, clang::SourceLocation, clang::CXXScopeSpec&, clang::IdentifierInfo*, clang::SourceLocation, clang::ParsedAttributesView const&, clang::AccessSpecifier, clang::SourceLocation, llvm::MutableArrayRef, bool&, bool&, clang::SourceLocation, bool, clang::ActionResult, false>, bool, bool, clang::OffsetOfKind, clang::SkipBodyInfo*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7178280)
#12 0x0000000006e125b2 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+++0x6e125b2)
#13 0x0000000006de6f36 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+++0x6de6f36)
#14 0x0000000006da1cd7 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6da1cd7)
#15 0x0000000006da275f clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6da275f)
#16 0x0000000006daa191 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6daa191)
#17 0x0000000006dab0a5 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dab0a5)
#18 0x0000000006d8c05a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d8c05a)
#19 0x0000000004fe80e8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fe80e8)
#20 0x0000000005301875 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5301875)
#21 0x000000000528164e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x528164e)
#22 0x00000000053fe5dd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x53fe5dd)
#23 0x0000000000ddc952 cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xddc952)
#24 0x0000000000dd328a ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#25 0x0000000000dd340d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#26 0x0000000005051819 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#27 0x0000000004423af4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4423af4)
#28 0x0000000005051e46 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#29 0x0000000005010cc2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5010cc2)
#30 0x0000000005011c6e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5011c6e)
#31 0x00000000050198c5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x50198c5)
#32 0x0000000000dd8cb4 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdd8cb4)
#33 0x0000000000c8ff1a main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc8ff1a)
#34 0x000076211a229d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#35 0x000076211a229e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#36 0x0000000000dd2d25 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdd2d25)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 134
```