[sanitizer] Fix the parent liveness check in StopTheWorld.

Comparing the parent PID with 1 isn't sufficient to ensure the parent is alive,
because of prctl(PR_SET_CHILD_SUBREAPER, ...). Compare with the real parent's
recorded PID instead.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@192295 91177308-0d34-0410-b5e6-96231b3b80d8