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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

You can navigate the links in the comments posted above, the failure that got me here was: https://lab.llvm.org/buildbot/#/builders/153/builds/2695

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