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

[Shafik Yaghmour via cfe-commits]

shafik wrote:

> So I believe:
> 
> ```
> typedef enum EnumA {
>   A
> } EnumA;
> 
> enum EnumB {
>   B,
>   B1 = 1,
>   B2 = A == B1
> };
> ```
> 
> is not an enum compare warning in C++ because `B1` doesn't have an enumeration type due to the enumeration not being fully-defined, and is not an enum compare warning in C because `A` has type `int` (due to p15) and `B1` has type `int` (due to p12).

Right, so do we have a test for C++ that verifies that.

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