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

[Petr Hosek via llvm-commits]

petrhosek wrote:

> Bots are still broken right now, I’ll revert for now.

Do you have a link to the broken bots? I wasn't aware of any further issues.

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