CrashRecoveryContext: Add a simple POSIX implementation.

CrashRecoveryContext: Add a simple POSIX implementation.
 - This works, but won't handle crashes on stack overflow, or signals delivered
   to a thread other than the one that crashed. The latter is particular annoying
   on Darwin, because SIGABRT tends to go to the main thread.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109717 91177308-0d34-0410-b5e6-96231b3b80d8