tsan: speed up pthread_setname_np

pthread_setname_np does linear search over all thread descriptors
to map pthread_t to the thread descriptor. This has O(N^2) complexity
and becomes much worse in the new tsan runtime that keeps all ever
existed threads in the thread registry.
Replace linear search with direct access if pthread_setname_np
is called for the current thread (a very common case).

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D113916