CrashRecovery/Darwin: On Darwin, raise sends a signal to the main thread instead

CrashRecovery/Darwin: On Darwin, raise sends a signal to the main thread instead
of the current thread. This has the unfortunate effect that assert() and abort()
will end up bypassing our crash recovery attempts. We work around this for
anything in the same linkage unit by just defining our own versions of the
assert handler and abort.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111583 91177308-0d34-0410-b5e6-96231b3b80d8