[libc] Enable llvmlibc clang-tidy checks

Summary:
Add  clang-tidy for llvm-libc source directory.

Example of check in action:
```
/workspace/llvm-project/libc/src/string/strcpy.cpp:10:1: error: system include stdio.h not allowed [llvmlibc-restrict-system-libc-headers,-warnings-as-errors]
#include <stdio.h>
^~~~~~~~~~~~~~~~~~
```

Reviewers: sivachandra, abrachet

Reviewed By: sivachandra, abrachet

Subscribers: aheejin, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D76559