[JITLink] Skip debug sections in MachO objects.

Debug sections will not be linked into the final executable and may contain
ambiguous relocations*. Skipping them avoids both some unnecessary processing
cost and the hassle of dealing with the problematic relocations.

* E.g. __debug_ranges contains non-extern relocations to the end of functions
hat begin with named symbols. Under the usual rules for interpreting non-extern
relocations these will be incorrectly associated with the following block, or
no block at all (if there is a gap between one block and the next).