[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: ed8565cf0b64ea5e88cc94f321b1870bb105d09d changed *BlockSignals to
only block asynchronous signals, despite the name.
