[MachO] Finish moving fat header swap functions to MachO.h

This is a follow-up to r273479. At the time I wrote r273479 I didn't connect the dots that the functions I was adding had to exist somewhere. Turns out, they do. This finishes moving the functions to MachO.h.

Existing MachO fat header tests like test/tools/llvm-readobj/Inputs/macho-universal-archive.x86_64.i386 execute this code.

llvm-svn: 273502