[clang] Assertion failure at dllExportImportClassTemplateSpecialization for windows

The following code:

```cpp
template
class S {};
template class S; // win32-warning {{explicit instantiation definition is not exported without 'dllexport'}}
extern template class __declspec(dllexport) S;
template class S;
```

Resulted in this output:
```
/tmp/test.cpp:4:45: error: explicit instantiation declaration (with 'extern') follows explicit instantiation definition (without 'extern')
4 | extern template class __declspec(dllexport) S;
| ^
/tmp/test.cpp:3:1: note: explicit instantiation definition is here
3 | template class S; // win32-warning {{explicit instantiation definition is not exported without 'dllexport'}}
| ^
clang++: /home/fuzz/WorkSpace/fusion-fuzz/projects/clang/llvm-project/clang/lib/Sema/SemaTemplate.cpp:10231: void dllExportImportClassTemplateSpecialization(clang::Sema&, clang::ClassTemplateSpecializationDecl*): Assertion `A && "dllExportImportClassTemplateSpecialization called " "on Def without dllexport or dllimport"' 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: /home/fuzz/WorkSpace/fusion-fuzz/projects/clang/llvm-clang-install/bin/clang++ -fsyntax-only -Os -std=c++11 -fblocks -ffast-math -target x86_64-mingw32 -ffreestanding /tmp/test.cpp
1. /tmp/test.cpp:5:22: current parser token ';'
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0 clang++ 0x00005bccb08a8329 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 121
1 clang++ 0x00005bccb08a4fec llvm::sys::RunSignalHandlers() + 76
2 clang++ 0x00005bccb08a58a8 llvm::sys::CleanupOnSignal(unsigned long) + 216
3 clang++ 0x00005bccb07e7218
4 libc.so.6 0x00007a2a8202e520
5 libc.so.6 0x00007a2a820829bc pthread_kill + 300
6 libc.so.6 0x00007a2a8202e476 raise + 22
7 libc.so.6 0x00007a2a820147f3 abort + 211
8 libc.so.6 0x00007a2a8201471b
9 libc.so.6 0x00007a2a82025e96
10 clang++ 0x00005bccb3b3d3ef
11 clang++ 0x00005bccb3b69139 clang::Sema::ActOnExplicitInstantiation(clang::Scope*, clang::SourceLocation, clang::SourceLocation, unsigned int, clang::SourceLocation, clang::CXXScopeSpec const&, clang::OpaquePtr, clang::SourceLocation, clang::SourceLocation, llvm::MutableArrayRef, clang::SourceLocation, clang::ParsedAttributesView const&) + 5545
12 clang++ 0x00005bccb312917d clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) + 7757
13 clang++ 0x00005bccb30f84a5 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::LateParsedAttrList*, clang::ImplicitTypenameContext) + 2821
14 clang++ 0x00005bccb31d6069 clang::Parser::ParseDeclarationAfterTemplate(clang::DeclaratorContext, clang::Parser::ParsedTemplateInfo&, clang::ParsingDeclRAIIObject&, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) + 2361
15 clang++ 0x00005bccb31d6a08 clang::Parser::ParseExplicitInstantiation(clang::DeclaratorContext, clang::SourceLocation, clang::SourceLocation, clang::SourceLocation&, clang::ParsedAttributes&, clang::AccessSpecifier) + 184
16 clang++ 0x00005bccb31e284c clang::Parser::ParseDeclarationStartingWithTemplate(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&) + 492
17 clang++ 0x00005bccb3107bb1 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) + 641
18 clang++ 0x00005bccb30c066e clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) + 638
19 clang++ 0x00005bccb30c16ff clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) + 575
20 clang++ 0x00005bccb309e632 clang::ParseAST(clang::Sema&, bool, bool) + 530
21 clang++ 0x00005bccb173f061 clang::FrontendAction::Execute() + 65
22 clang++ 0x00005bccb16c8e65 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) + 1589
23 clang++ 0x00005bccb1816de3 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) + 467
24 clang++ 0x00005bccae67233e cc1_main(llvm::ArrayRef, char const*, void*) + 7166
25 clang++ 0x00005bccae66813a
26 clang++ 0x00005bccae6682cf
27 clang++ 0x00005bccb144dcfd
28 clang++ 0x00005bccb07e7630 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) + 160
29 clang++ 0x00005bccb144eb53
30 clang++ 0x00005bccb1403607 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const + 167
31 clang++ 0x00005bccb1407ef0 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const + 304
32 clang++ 0x00005bccb1416004 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) + 404
33 clang++ 0x00005bccae66d923 clang_main(int, char**, llvm::ToolContext const&) + 7315
34 clang++ 0x00005bccae5a97f1 main + 113
35 libc.so.6 0x00007a2a82015d90
36 libc.so.6 0x00007a2a82015e40 __libc_start_main + 128
37 clang++ 0x00005bccae667765 _start + 37
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 24.0.0git (https://github.com/llvm/llvm-project.git 0976d75180f482c1175ea1f204f9bfd56dd163b1)
Target: x86_64-unknown-windows-gnu
Thread model: posix
InstalledDir: /home/fuzz/WorkSpace/fusion-fuzz/projects/clang/llvm-clang-install/bin
Build config: +assertions
clang++: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING CRASH REPRODUCER FILES TO THE BUG REPORT:
clang++: note: diagnostic msg: /tmp/test-2bd060.cpp
clang++: note: diagnostic msg: /tmp/test-2bd060.sh
clang++: note: diagnostic msg:

********************
Aborted (core dumped)
```

To reproduce:
```
/home/fuzz/WorkSpace/fusion-fuzz/projects/clang/llvm-clang-install/bin/clang++ -target x86_64-mingw32 ./test.cpp
```

Reproduced in trunk: https://godbolt.org/z/YsfsbfPPc

It goes back to clang-9: https://godbolt.org/z/xhsY4bM9M

Compiler version:
```
clang version 24.0.0git (https://github.com/llvm/llvm-project.git 0976d75180f482c1175ea1f204f9bfd56dd163b1)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/fuzz/WorkSpace/fusion-fuzz/projects/clang/llvm-clang-install/bin
Build config: +assertions
```

*This bug was found by [fusion-fuzz](https://github.com/fusion-fuzz/fusion-fuzz)*

Start with the reproducer in test.cpp and run it using the reported x86_64-mingw32 command. Then inspect clang/lib/Sema/SemaTemplate.cpp around dllExportImportClassTemplateSpecialization and the explicit-instantiation path shown in the stack trace. Done means the reproducer reports the appropriate diagnostic without triggering an assertion failure.

Written by the indexing model from the issue text.