[clang] Assertion `getCommonPtr()->InstantiatedFromMember.getPointer() && "Only member templates can be member template specializations"' failed.

Reproducer:
https://godbolt.org/z/E7ef66Yjc
```cpp
template struct S {};
template <> template int S::foo;
template <> template int S::foo;
template <> template constexpr int S::foo;
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/tools/clang/include/clang/AST/DeclTemplate.h:858: void clang::RedeclarableTemplateDecl::setMemberSpecialization(): Assertion `getCommonPtr()->InstantiatedFromMember.getPointer() && "Only member templates can be member template specializations"' failed.
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. :4:61: current parser token ';'
#0 0x00000000043f3668 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f3668)
#1 0x00000000043f0524 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f0524)
#2 0x00000000043f0b4c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43f0b4c)
#3 0x000000000432fb88 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x00007936aa645330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x00007936aa69eb2c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb2c)
#6 0x00007936aa64527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x00007936aa6288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x00007936aa62881b (/lib/x86_64-linux-gnu/libc.so.6+0x2881b)
#9 0x00007936aa63b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#10 0x00000000070f5ecb clang::Sema::ActOnVariableDeclarator(clang::Scope*, clang::Declarator&, clang::DeclContext*, clang::TypeSourceInfo*, clang::LookupResult&, llvm::MutableArrayRef, bool&, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x70f5ecb)
#11 0x0000000007110777 clang::Sema::HandleDeclarator(clang::Scope*, clang::Declarator&, llvm::MutableArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7110777)
#12 0x0000000007718e6e clang::Sema::ActOnTemplateDeclarator(clang::Scope*, llvm::MutableArrayRef, clang::Declarator&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7718e6e)
#13 0x0000000006d37dde clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d37dde)
#14 0x0000000006d587f9 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+++0x6d587f9)
#15 0x0000000006e1d180 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e1d180)
#16 0x0000000006e297f3 clang::Parser::ParseTemplateDeclarationOrSpecialization(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) (.part.0) ParseTemplate.cpp:0:0
#17 0x0000000006e29bea clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e29bea)
#18 0x0000000006d5a2b0 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d5a2b0)
#19 0x0000000006d10df6 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d10df6)
#20 0x0000000006d11ce5 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d11ce5)
#21 0x0000000006ceebfa clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6ceebfa)
#22 0x0000000004ef1628 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ef1628)
#23 0x000000000521ccc8 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x521ccc8)
#24 0x00000000051aa16d clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x51aa16d)
#25 0x00000000052f53fd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52f53fd)
#26 0x0000000000da6c9c cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda6c9c)
#27 0x0000000000d9d8aa ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#28 0x0000000000d9da2d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#29 0x0000000004f688f9 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#30 0x000000000432ffc4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x432ffc4)
#31 0x0000000004f69465 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#32 0x0000000004f21e32 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f21e32)
#33 0x0000000004f26667 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f26667)
#34 0x0000000004f30bc5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f30bc5)
#35 0x0000000000da318a clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda318a)
#36 0x0000000000cbef4a main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcbef4a)
#37 0x00007936aa62a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#38 0x00007936aa62a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#39 0x0000000000d9d255 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd9d255)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 134
```

Run the linked Godbolt reproducer first, then inspect the assertion in llvm/tools/clang/include/clang/AST/DeclTemplate.h and the reported clang::Sema::ActOnVariableDeclarator entry point. Determine how the repeated template declarations reach this assertion; done means the reproducer no longer aborts while preserving the intended diagnostic or compilation behavior.

Written by the indexing model from the issue text.