tsan: fix ignore handling in signal handlers

We've reset thr->ignore_reads_and_writes, but forget to do
thr->fast_state.ClearIgnoreBit(). So ignores were not effective
reset and fast_state.ignore_bit was corrupted if signal handler
itself uses ignores.

Properly reset/restore fast_state.ignore_bit around signal handlers.

llvm-svn: 265288