[Support] Use thread safe version of getpwuid and getpwnam.

OpenGroup specification doesn't require getpwuid and getpwnam
to be thread-safe. And musl libc has a not thread-safe implementation.
When building clang with musl, this can make clang-scan-deps crash.

Reviewed By: pirama

Differential Revision: https://reviews.llvm.org/D137864