[clang-tidy] Add module for llvm-libc and restrict-system-libc-header-check.

Summary: This adds a new module to enforce standards specific to the llvm-libc project. This change also adds the first check which restricts user from including system libc headers accidentally which can lead to subtle bugs that would be a challenge to detect.

Reviewers: alexfh, hokein, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: juliehockett, arphaman, jfb, abrachet, sivachandra, Eugene.Zelenko, njames93, mgorny, xazax.hun, MaskRay, cfe-commits

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

Differential Revision: https://reviews.llvm.org/D75332