[clang] objective-c++ assertion failure at clang::Parser::ParseObjCMethodDefinition

The following code:

```objective-c++
@implementation ISDPropertyChangeGroup
extern "C" {
@end
};
- (id)lastModifiedGeneration : (ISDClientState *) obj
{}
```

Resulted in this output:
```
/tmp/test.mm:1:17: warning: cannot find interface declaration for 'ISDPropertyChangeGroup'
1 | @implementation ISDPropertyChangeGroup
| ^
/tmp/test.mm:5:33: error: expected a type
5 | - (id)lastModifiedGeneration : (ISDClientState *) obj
| ^
clang++: /home/fuzz/WorkSpace/fusion-fuzz/projects/clang/llvm-project/clang/lib/Parse/ParseObjc.cpp:2469: clang::Decl* clang::Parser::ParseObjCMethodDefinition(): Assertion `CurParsedObjCImpl && "ParseObjCMethodDefinition - Method out of @implementation"' 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.mm
1. /tmp/test.mm:6:1: current parser token '{'
2. /tmp/test.mm:6:1: parsing Objective-C method 'ISDPropertyChangeGroup::lastModifiedGeneration:'
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++ 0x00005564885340f9 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 121
1 clang++ 0x0000556488530dcc llvm::sys::RunSignalHandlers() + 76
2 clang++ 0x0000556488531678 llvm::sys::CleanupOnSignal(unsigned long) + 216
3 clang++ 0x0000556488473f88
4 libc.so.6 0x00007fbc92e67520
5 libc.so.6 0x00007fbc92ebb9fc pthread_kill + 300
6 libc.so.6 0x00007fbc92e67476 raise + 22
7 libc.so.6 0x00007fbc92e4d7f3 abort + 211
8 libc.so.6 0x00007fbc92e4d71b
9 libc.so.6 0x00007fbc92e5ee96
10 clang++ 0x000055648ad8493b clang::Parser::ParseObjCMethodDefinition() + 1867
11 clang++ 0x000055648acd3f4b clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) + 2939
12 clang++ 0x000055648acd47df clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) + 575
13 clang++ 0x000055648acb170a clang::ParseAST(clang::Sema&, bool, bool) + 586
14 clang++ 0x000055648922a071 clang::FrontendAction::Execute() + 65
15 clang++ 0x00005564891b3c65 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) + 1589
16 clang++ 0x0000556489305ea3 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) + 467
17 clang++ 0x0000556486f08c96 cc1_main(llvm::ArrayRef, char const*, void*) + 7046
18 clang++ 0x0000556486efea2a
19 clang++ 0x0000556486efebbf
20 clang++ 0x0000556488f3b35d
21 clang++ 0x00005564884743a0 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) + 160
22 clang++ 0x0000556488f3c1b3
23 clang++ 0x0000556488ef1987 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const + 167
24 clang++ 0x0000556488ef61e0 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const + 304
25 clang++ 0x0000556488f03e44 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) + 404
26 clang++ 0x0000556486f042d3 clang_main(int, char**, llvm::ToolContext const&) + 7267
27 clang++ 0x0000556486e567a1 main + 113
28 libc.so.6 0x00007fbc92e4ed90
29 libc.so.6 0x00007fbc92e4ee40 __libc_start_main + 128
30 clang++ 0x0000556486efe055 _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-a583e6.mm
clang++: note: diagnostic msg: /tmp/test-a583e6.sh
clang++: note: diagnostic msg:

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

```

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

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.mm and inspect clang::Parser::ParseObjCMethodDefinition in clang/lib/Parse/ParseObjC.cpp, especially the assertion reported at line 2469. Run the supplied clang++ command with assertions enabled; done means the Objective-C++ input no longer crashes and has an appropriate diagnostic or recovery behavior.

Written by the indexing model from the issue text.