[clang] [C++20][Modules] Improve namespace look-up performance for modules. (PR #171769)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> For us, it's because at a certain scale with Thrift generated files, we end up importing 1K+ modules that all declare the apache::thrift namespace.

Maybe this is a nice point of named modules. We use thrift too. But we just wrap it into a single named module. Then we avoid the problem.

https://github.com/llvm/llvm-project/pull/171769