[llvm-objdump] Suppress spurious warnings when parsing Mach-O binaries.

Summary:
llvm-objdump started warning when asked to disassemble a section that
isn't present in the input files, in Yuanfang Chen's change:
d16c162c. The problem is that the
logic was restricted only to the generic llvm-objdump parser, not to the
Mach-O-specific parser used for Apple toolchain compatibility. The
solution is to log section names from the Mach-O parser.

The macho-cstring-dump.test has been updated to fail if it encounters
this new warning in the future.

Reviewers: pete, ab, lhames, jhenderson, grimar, MaskRay, ychen

Reviewed By: jhenderson, grimar

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73586