[dwarfdump] Ignore scattered relocations for mach-o.

[dwarfdump] Ignore scattered relocations for mach-o.

When encountering a scattered relocation, the code would assert trying to
access an unexisting section. I couldn't find a way to expose the result
of the processing of a scattered reloc, and I'm really unsure what the
right thing to do is. This patch just skips them during the processing in
DwarfContext and adds a mach-o file to the tests that exposed the asserting
behavior.
(This is a new failure that is being exposed by Rafael's recent work on
the libObject interfaces. I think the wrong behavior has always happened,
but now it's asserting)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243778 91177308-0d34-0410-b5e6-96231b3b80d8