[libc] Fix accidental inclusion of system libc headers.

Summary:
I found that because `--system-headers` flag was not included when running clang-tidy, errors produced from compiler provided headers were being suppressed. After passing this flag I realized that by including headers like stdint.h we were indirectly including headers from the system libc. To prevent this we pass `-ffreestanding`.
We don't want to pass `--system-headers` for all checks just the `llvmlibc-restrict-system-libc-headers` therefore we do a separate invocation of clang-tidy for this check.

Reviewers: abrachet, sivachandra

Reviewed By: sivachandra

Subscribers: mgorny, aheejin, tschuett, ecnelises, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D80178