[msan] Make origin tracking fork-safe.

Chained origins make plain memory stores async-signal-unsafe.
We already disable it inside signal handlers.
This change grabs all origin-related locks before fork() and releases
them after fork() to avoid a deadlock in the child process.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@217140 91177308-0d34-0410-b5e6-96231b3b80d8