[clang] [Clang][Sema] Fix type of enumerators in incomplete enumerations (PR #84068)

[via cfe-commits]

Kupa-Martin wrote:

> > @shafik We dont have a dedicated cpp test for this. I can add one if you want, but clang/test/Sema/warn-compare-enum-types-mismatch.c runs clang both on C and C++ mode, so I didnt think it necessary.
> 
> I think we just a bug that demonstrates this issue: #84712
> 
> So if this fixes that as well then we should add tests for those cases too.

I've included a separate test for C++. If there are no more changes, could someone please merge this for me? I dont have commit access.

https://github.com/llvm/llvm-project/pull/84068