tsan: simplify thread context setting

[dvyukov]

Summary

Currently we set thr->tctx after OnStarted callback
taking thread registry mutex again and searching for the context.
But OnStarted already runs under the thread registry mutex
and has access to the context, so set it in the OnStarted.
This makes code simpler and faster.

Depends on D110132.

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

if (tid != kMainTid) {
     if (tls_addr && tls_size) ImitateTlsWrite(thr, tls_addr, tls_size);
   }
 #endif
 
   ThreadRegistry *tr = &ctx->thread_registry;
   OnStartedArgs args = { thr, stk_addr, stk_size, tls_addr, tls_size };
   tr->StartThread(tid, os_id, thread_type, &args);
 
-  tr->Lock();
-  thr->tctx = (ThreadContext*)tr->GetThreadLocked(tid);
-  tr->Unlock();
-
   while (!thr->tctx->trace.parts.Empty()) thr->tctx->trace.parts.PopBack();
 
 #if !SANITIZER_GO
   if (ctx->after_multithreaded_fork) {
     thr->ignore_interceptors++;
     ThreadIgnoreBegin(thr, 0);
     ThreadIgnoreSyncBegin(thr, 0);
   }
#endif
   thr->fast_state.SetHistorySize(flags()->history_size);
   // Commit switch to the new part of the trace.
   // TraceAddEvent will reset stack0/mset0 in the new part for us.
   TraceAddEvent(thr, thr->fast_state, EventTypeMop, 0);
 
   thr->fast_synch_epoch = epoch0;
   AcquireImpl(thr, 0, &sync);
   sync.Reset(&thr->proc()->clock_cache);
+  thr->tctx = this;
   thr->is_inited = true;
   DPrintf(
       "#%d: ThreadStart epoch=%zu stk_addr=%zx stk_size=%zx "
       "tls_addr=%zx tls_size=%zx\n",
       tid, (uptr)epoch0, args->stk_addr, args->stk_size, args->tls_addr,
       args->tls_size);
 }