[msan] Block signals in MsanThread::Init

If async signal handler called when we MsanThread::Init
signal handler may trigger false reports.
I failed to reproduce this locally for a test.

Differential Revision: https://reviews.llvm.org/D113328

GitOrigin-RevId: f2c2292fa801896751cadec4f65bef88bf83ba5b
2 files changed