[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

test/DebugInfo/Inputs/dwarfdump-test.cc

@@ -27,3 +27,5 @@ int main() {
 // $ cp <output> output2.dwz
 // $ dwz -m output.dwz -r output1.dwz output2.dwz
 // $ rm output2.dwz
+
+// The mach-o version was generated using clang-3.6.2.

test/DebugInfo/dwarfdump-dump-flags.test

@@ -1,6 +1,9 @@
 ; RUN: llvm-dwarfdump %p/Inputs/dwarfdump-test.elf-x86-64 -debug-dump=all | FileCheck %s -check-prefix DUMP_ALL
 ; RUN: llvm-dwarfdump %p/Inputs/dwarfdump-test.elf-x86-64 -debug-dump=info | FileCheck %s -check-prefix DUMP_INFO
 ; RUN: llvm-dwarfdump %p/Inputs/dwarfdump-test.elf-x86-64 -debug-dump=ranges | FileCheck %s -check-prefix DUMP_RANGES
+; RUN: llvm-dwarfdump %p/Inputs/dwarfdump-test.macho-i386.o -debug-dump=all | FileCheck %s -check-prefix DUMP_ALL
+; RUN: llvm-dwarfdump %p/Inputs/dwarfdump-test.macho-i386.o -debug-dump=info | FileCheck %s -check-prefix DUMP_INFO
+; RUN: llvm-dwarfdump %p/Inputs/dwarfdump-test.macho-i386.o -debug-dump=ranges | FileCheck %s -check-prefix DUMP_RANGES
 
 ; DUMP_ALL: .debug_info
 ; DUMP_ALL: .debug_ranges