[TSan][Darwin] Avoid calling pthread_self() before libpthread is initialized

[yln]

Summary

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

compiler-rt/lib/tsan/rtl/tsan_platform_mac.cpp

 // shadow memory of the pointer returned by pthread_self() to store a pointer to
 // the ThreadState object. The main thread's ThreadState is stored separately
 // in a static variable, because we need to access it even before the
 // shadow memory is set up.
 static uptr main_thread_identity = 0;
 ALIGNED(64) static char main_thread_state[sizeof(ThreadState)];
 static ThreadState *main_thread_state_loc = (ThreadState *)main_thread_state;
 
+// We cannot use pthread_self() before libpthread has been initialized.  Our
+// current heuristic for guarding this is checking `main_thread_identity` which
+// is only assigned in `__tsan::InitializePlatform`.
 static ThreadState **cur_thread_location() {
+  if (main_thread_identity == 0)
+    return &main_thread_state_loc;
   uptr thread_identity = (uptr)pthread_self();
-  if (thread_identity == main_thread_identity || main_thread_identity == 0)
+  if (thread_identity == main_thread_identity)
     return &main_thread_state_loc;
   return (ThreadState **)MemToShadow(thread_identity);
 }
 
 ThreadState *cur_thread() {
   return (ThreadState *)SignalSafeGetOrAllocate(
       (uptr *)cur_thread_location(), sizeof(ThreadState));
 }