[mach-o] Add support for interposing tuples section

Darwin uses two-level-namespace lookup for symbols which means the static
linker records where each symbol must be found at runtime.  Thus defining a
symbol in a dylib loaded earlier will not effect where symbols needed by
later dylibs will be found.  Instead overriding is done through a section
of type S_INTERPOSING which contains tuples of <interposer, interposee>.

llvm-svn: 221421