[lld-macho] Don't attempt to emit rebase opcodes for debug sections

This was causing a crash as we were attempting to look up the
nonexistent parent OutputSection of the debug sections. We didn't detect
it earlier because there was no test for PIEs with debug info (PIEs
require us to emit rebases for X86_64_RELOC_UNSIGNED).

This diff filters out the debug sections while loading the ObjFiles. In
addition to fixing the above problem, it also lets us avoid doing
redundant work -- we no longer parse / apply relocations / attempt to
emit dyld opcodes for these sections that we don't emit.

Fixes llvm.org/PR48392.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D92904