[libc-commits] [libc] [libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration (PR #98075)

[Petr Hosek via libc-commits]

petrhosek wrote:

I used `clang-include-cleaner` to add the `#include`s although I think this may have been too eager and it'd be worthwhile running https://github.com/llvm/llvm-project/blob/d3cb277ea35d2cf25cdcc41b894959332477861c/clang-tools-extra/clang-tidy/misc/IncludeCleanerCheck.cpp to remove the unnecessary ones.

https://github.com/llvm/llvm-project/pull/98075