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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

I don't feel the current PR quite good as it introduces some redundant logics with `Sema::makeMergedDefinitionVisible(NamedDecl *ND)` and given @avikivity said he doesn't understand the code and according to our AI policy, contributors should understand their code. 

I'd like to create another PR https://github.com/llvm/llvm-project/pull/189903 for maintaining considerations and close the current one.

And after all, this PR saved me a lot of time to understand what happened. Thanks for your effort.

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