[msan] Block signals during MsanThread::TSDDtor (#98405)

MSan may segfault inside a signal handler, if MSan instrumentation is
trying to access thread-local storage that has already been destroyed.
This fixes the issue by blocking asychronous signals inside
MsanThread::TSDDtor. This is based on an idea suggested by Paul
Pluzhnikov (block async signals in MsanThread::Destroy()) and refined by
Vitaly Buka.
    
Note: ed8565c changed *BlockSignals to
only block asynchronous signals, despite the name.

Author: thurstond

compiler-rt/lib/msan/msan_linux.cpp

@@ -292,6 +292,7 @@ void MsanTSDDtor(void *tsd) {
     CHECK_EQ(0, pthread_setspecific(tsd_key, tsd));
     return;
   }
+  ScopedBlockSignals block(nullptr);
   msan_current_thread = nullptr;
   // Make sure that signal handler can not see a stale current thread pointer.
   atomic_signal_fence(memory_order_seq_cst);