[clang] Fix enum enumerator visibility loss when #include is between module imports (PR #188865)

[Avi Kivity via cfe-commits]

avikivity wrote:

> @avikivity I see an undesired side effect: enum redefinition is no longer diagnosed if the first declaration was in a module.
> 
> Consider:
> 
> ```c++
> // modA.ccpm
> module;
> enum Enum {
>     ENUM
> };
> export module modA;
> ```
> 
> ```c++
> // main.cc
> import modA;
> enum Enum {
>     ENUM
> };
> enum Enum {
>     ENUM
> };
> ```
> 
> Before your change, this will (desirably) fail with a `error: redefinition of 'Enum'`. After your change, it will compile.

Clang 22.1.1 (unpatched) also does not reject the redefinition. The patch does not cause a regression, and it is yet another bug.

https://github.com/llvm/llvm-project/pull/188865