[clang-tools-extra] [clang-tidy] Update llvmlibc-implementation-in-namespace to new rules (PR #66504)

[Guillaume Chatelet via cfe-commits]

gchatelet wrote:

> As a single small change to make hardcoded namespaces configurable looks fine. I added some comments related to overall issues in this check. Fell free to fix them or ignore them.

Thx for the review! I had to restructure the code a bit to accommodate for anonymous namespaces (they generate an implict UsingDirectiveDecl that was triggering the warning twice).

Let me know what you think.

https://github.com/llvm/llvm-project/pull/66504