[libc][clang-tidy] fix namespace check for externals

Up until now, all references to `errno` were marked with `NOLINT`, since
it was technically calling an external function. This fixes the lint
rules so that `errno`, as well as `malloc`, `calloc`, `realloc`, and
`free` are all allowed to be called as external functions. All of the
relevant `NOLINT` comments have been removed, and the documentation has
been updated.

Reviewed By: sivachandra, lntue, aaron.ballman

Differential Revision: https://reviews.llvm.org/D113946