This skips calling `pthread_self` when `main_thread_identity` hasn't
been initialized yet. `main_thread_identity` is only ever assigned in
`__tsan::InitializePlatform`. This change should be relatively safe; we
are not changing behavior other than skipping the call to `pthread_self`
when `main_thread_identity == 0`.
rdar://57822138
Reviewed By: kubamracek
Differential Revision: https://reviews.llvm.org/D71559
