[sanitizer] Introduce tid_t as a typedef for OS-provided thread IDs

We seem to assume that OS-provided thread IDs are either uptr or int, neither of which is true on Darwin. This introduces a tid_t type, which holds a OS-provided thread ID (gettid on Linux, pthread_threadid_np on Darwin, pthread_self on FreeBSD).

Differential Revision: https://reviews.llvm.org/D31774

llvm-svn: 300473