[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

libc/src/.clang-tidy

@@ -0,0 +1,6 @@
+Checks: '-*,llvmlibc-*'
+HeaderFilterRegex: '.*'
+WarningsAsErrors: 'llvmlibc-*'
+CheckOptions:
+  - key:             llvmlibc-restrict-system-libc-headers.Includes
+    value:           '-*, linux/*, asm/unistd.h'

libc/src/math/round_redirector.cpp

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// Include okay for this redirector.
+// NOLINTNEXTLINE(llvmlibc-restrict-system-libc-headers)
 #include <math.h>
 
 namespace __llvm_libc {