[clang-tidy] Add check callee-namespace.

Summary:
This check will ensure that all calls to functions resolve to one inside the `__llvm_libc` namespace.

This is done to ensure that if we include a public header then we don't accidentally call into the a function within the global namespace.

Reviewers: aaron.ballman, njames93

Reviewed By: aaron.ballman

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

Tags: #clang-tools-extra, #libc-project, #clang

Differential Revision: https://reviews.llvm.org/D78890