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

[Shafik Yaghmour via cfe-commits]

shafik 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: https://github.com/llvm/llvm-project/issues/84712

So if this fixes that as well then we should add tests for those cases too.

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