[clang] [Serialization] [ASTWriter] Try to not record namespace as much as possible (PR #179178)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Just found out this can fix https://github.com/llvm/llvm-project/issues/184957 

And the implementation is already tested by @mpark for header modules. Also from the implementors' perspective, I believe it is correct. 

The only concerning point is about the performance. But whether it is a problem or not is highly relates to the structure of the module being used. In my mind, it may be a problem in case there are thousands of duplicated modules,  which majorly is for implicit modules. So from maintainability, I'd like to unify the code to remove the interface. But this requires the testing result from @jtstogel . I'll open a new PR for you.

I'll land this PR after polishing it.

https://github.com/llvm/llvm-project/pull/179178