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

[Cameron Angus via cfe-commits]

kamrann wrote:

> Do you have an exact command line invocation? You're presumably using some form of modules?

Yes I'm seeing this with c++20 modules. Just extracted this from my build system, seems to suffice:
```
clang++ -std=c++2b -stdlib=libc++ -Xclang -fmodules-embed-all-files -fmodule-output=out/std.so.pcm -o out/std.so.pcm.o -c -x c++-module [[llvm-installation]]/share/libc++/v1/std.cppm
clang++ -std=c++2b -stdlib=libc++ -fmodule-file=std=out/std.so.pcm -o out/repro.a.o -c -x c++ repro.cpp
```

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