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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@jtstogel @mpark hi, would you like to test this? I'd like to land this in 6 weeks if no more comments came in.

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