Reapply "[ORC] Initial MachO debugging support (via GDB JIT debug.." with fixes.

This reapplies e1933a04 (which was reverted in
f55ba352 due to bot failures, e.g.
https://lab.llvm.org/buildbot/#/builders/117/builds/2768).

The bot failures were due to a missing symbol error: We use the input object's
mangling to decide how to mangle the debug-info registration function name. This
caused lookup of the registration function to fail when the input object
mangling didn't match the host mangling.

Disbaling the test on non-Darwin platforms is the easiest short-term solution.
I have filed https://llvm.org/PR52503 with a proposed longer term solution.