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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Although I can use the reproducer "import std;" to verify offline.

A minimal reproducer can be summitted in tree and help us to improve the quality of testing.

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