Add RelocVisitor support for MachO

This commit adds partial support for MachO relocations to RelocVisitor.
A simple test case is added to show that relocations are indeed being
applied and that using llvm-dwarfdump on MachO files no longer errors.
Correctness is not yet tested, due to an unrelated bug in DebugInfo,
which will be fixed with appropriate testcase in a followup commit.

Differential Revision: http://reviews.llvm.org/D8148

llvm-svn: 238663