[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