[llvm] [NFC] Eliminate top-level "using namespace" from some headers. (PR #102751)

[Oliver Stöneberg via llvm-commits]

firewave wrote:

> Also, I wonder if there is a clang-tidy warning for this. I'll try to find.

There is `google-build-using-namespace`: https://clang.llvm.org/extra/clang-tidy/checks/google/build-using-namespace.html. That flags *all* usages though. It (or an alias) should probably get an option to limit it to headers only.

https://github.com/llvm/llvm-project/pull/102751