[clang-tidy] Add check llvmlibc-implementation-in-namespace.

Summary:
This check makes sure all llvm-libc implementations falls within the `__llvm_libc` namespace.

Reviewers: alexfh, aaron.ballman, hokein, njames93

Reviewed By: aaron.ballman

Subscribers: Eugene.Zelenko, libc-commits, mgorny, xazax.hun, MaskRay, cfe-commits, sivachandra

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D76818