Resubmit r295469 [PGO] Suspend SIGKILL for PR_SET_PDEATHSIG in profile-write

And also r295364 [PGO] remove unintended debug trace. NFC
I removed the test case: it's hard to write synchronized test b/w processes
in this framework. I will revisit the test-case later.

llvm-svn: 298113