[clang] Fix clang reject valid C++ code after d999ce0302f06d250f6d496b56a5a5f (PR #94471)

[Haojian Wu via cfe-commits]

hokein wrote:

unfortunately, this seems to break an existing test:

```
       OK ] InterpreterTest.IncrementalInputTopLevelDecls (66 ms)
[ RUN      ] InterpreterTest.Errors
ClangReplInterpreterTests: /usr/local/google/home/hokein/workspace/llvm-project/clang/lib/Sema/IdentifierResolver.cpp:228: void clang::IdentifierResolver::RemoveDecl(NamedDecl *): Assertion `Ptr && "Didn't find this decl on its identifier's chain!"' failed.
 #0 0x00005591835285e1 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (tools/clang/unittests/Interpreter/ClangReplInterpreterTests+0xa4e45e1)
 #1 0x0000559183528b9b PrintStackTraceSignalHandler(void*) Signals.cpp:0:0
 #2 0x0000559183526356 llvm::sys::RunSignalHandlers() (tools/clang/unittests/Interpreter/ClangReplInterpreterTests+0xa4e2356)
 #3 0x0000559183529d35 SignalHandler(int) Signals.cpp:0:0
 #4 0x00007f5baf25a510 (/lib/x86_64-linux-gnu/libc.so.6+0x3c510)
 #5 0x00007f5baf2a816
```

https://github.com/llvm/llvm-project/pull/94471