[clang] assertion failure at clang::Sema::ActOnFinishCXXInClassMemberInitializer

https://godbolt.org/z/GoKPhbfK8

The following code:

```cpp
union Union { int i; float f;
template<> struct PotentiallySealed sealed { }
}
```

Resulted in this output:
```
/tmp/test.cpp:3:8: error: explicit specialization of undeclared template struct 'PotentiallySealed'
3 | struct PotentiallySealed sealed { }
| ^ ~~~~~
/tmp/test.cpp:2:2: error: extraneous 'template<>' in declaration of variable 'sealed'
2 | template<>
| ^~~~~~~~~~
/tmp/test.cpp:3:41: error: expected ';' at end of declaration list
3 | struct PotentiallySealed sealed { }
| ^
| ;
/tmp/test.cpp:3:41: error: expected '}'
/tmp/test.cpp:1:13: note: to match this '{'
1 | union Union { int i; float f;
| ^
clang++: /home/fuzz/WorkSpace/fusion-fuzz/projects/clang/llvm-project/clang/lib/Sema/SemaDeclCXX.cpp:4241: void clang::Sema::ActOnFinishCXXInClassMemberInitializer(clang::Decl*, clang::SourceLocation, clang::ExprResult): Assertion `(FD && FD->getInClassInitStyle() != ICIS_NoInit) && "must set init style when field is created"' 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++ -S -o /dev/null -O3 -std=c++20 /tmp/test.cpp
1. parser at end of file
2. /tmp/test.cpp:1:1: parsing struct/union/class body 'Union'
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++ 0x000055e2892730f9 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 121
1 clang++ 0x000055e28926fdcc llvm::sys::RunSignalHandlers() + 76
2 clang++ 0x000055e289270678 llvm::sys::CleanupOnSignal(unsigned long) + 216
3 clang++ 0x000055e2891b2f88
4 libc.so.6 0x00007f3bf2e11520
5 libc.so.6 0x00007f3bf2e659fc pthread_kill + 300
6 libc.so.6 0x00007f3bf2e11476 raise + 22
7 libc.so.6 0x00007f3bf2df77f3 abort + 211
8 libc.so.6 0x00007f3bf2df771b
9 libc.so.6 0x00007f3bf2e08e96
10 clang++ 0x000055e28be71637 clang::Sema::ActOnFinishCXXInClassMemberInitializer(clang::Decl*, clang::SourceLocation, clang::ActionResult) + 263
11 clang++ 0x000055e28ba2275c
12 clang++ 0x000055e28ba21f2d clang::Parser::ParseLexedMemberInitializers(clang::Parser::ParsingClass&) + 397
13 clang++ 0x000055e28ba78a86 clang::Parser::ParseCXXMemberSpecification(clang::SourceLocation, clang::SourceLocation, clang::ParsedAttributes&, unsigned int, clang::Decl*) + 1542
14 clang++ 0x000055e28ba7b4ff clang::Parser::ParseClassSpecifier(clang::tok::TokenKind, clang::SourceLocation, clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, bool, clang::Parser::DeclSpecContext, clang::ParsedAttributes&) + 8191
15 clang++ 0x000055e28ba4a732 clang::Parser::ParseDeclarationSpecifiers(clang::DeclSpec&, clang::Parser::ParsedTemplateInfo&, clang::AccessSpecifier, clang::Parser::DeclSpecContext, clang::LateParsedAttrList*, clang::ImplicitTypenameContext) + 2994
16 clang++ 0x000055e28ba05e88 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) + 200
17 clang++ 0x000055e28ba0692f clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) + 959
18 clang++ 0x000055e28ba127a1 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) + 977
19 clang++ 0x000055e28ba137df clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) + 575
20 clang++ 0x000055e28ba13bf4 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) + 68
21 clang++ 0x000055e28b9f06db clang::ParseAST(clang::Sema&, bool, bool) + 539
22 clang++ 0x000055e289f69071 clang::FrontendAction::Execute() + 65
23 clang++ 0x000055e289ef2c65 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) + 1589
24 clang++ 0x000055e28a044ea3 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) + 467
25 clang++ 0x000055e287c47c96 cc1_main(llvm::ArrayRef, char const*, void*) + 7046
26 clang++ 0x000055e287c3da2a
27 clang++ 0x000055e287c3dbbf
28 clang++ 0x000055e289c7a35d
29 clang++ 0x000055e2891b33a0 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) + 160
30 clang++ 0x000055e289c7b1b3
31 clang++ 0x000055e289c30987 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const + 167
32 clang++ 0x000055e289c351e0 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const + 304
33 clang++ 0x000055e289c42e44 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) + 404
34 clang++ 0x000055e287c432d3 clang_main(int, char**, llvm::ToolContext const&) + 7267
35 clang++ 0x000055e287b957a1 main + 113
36 libc.so.6 0x00007f3bf2df8d90
37 libc.so.6 0x00007f3bf2df8e40 __libc_start_main + 128
38 clang++ 0x000055e287c3d055 _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 aefba88f46a6e55645c848f58f6ba56944d5ae62)
Target: x86_64-unknown-linux-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-4b09dc.cpp
clang++: note: diagnostic msg: /tmp/test-4b09dc.sh
clang++: note: diagnostic msg:

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

```

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

Compiler version:
```
clang version 24.0.0git (https://github.com/llvm/llvm-project.git aefba88f46a6e55645c848f58f6ba56944d5ae62)
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 the provided clang++ command with assertions enabled. Then inspect clang/lib/Sema/SemaDeclCXX.cpp, especially ActOnFinishCXXInClassMemberInitializer and the parser stack frames shown. Done means this malformed C++ input reports diagnostics without triggering the assertion or aborting.

Written by the indexing model from the issue text.