[Object] Extend MachOUniversalBinary::getObjectForArch

Make the method MachOUniversalBinary::getObjectForArch return MachOUniversalBinary::ObjectForArch
and add helper methods MachOUniversalBinary::getMachOObjectForArch, MachOUniversalBinary::getArchiveForArch
for those who explicitly expect to get a MachOObjectFile or an Archive.

Differential revision: https://reviews.llvm.org/D67700

Test plan: make check-all

llvm-svn: 372278