A particular slice of a universal binary can be either a MachO object or an archive
(although the class MachOUniversalBinary doesn't store them explicitly, instead, they can be constructed "on demand"
via calling the method ObjectForArch::getAsObjectFile or ObjectForArch::getAsArchive)
Before this patch MachOUniversalBinary had only a method (getObjectForArch)
to look up a slice for a particular architecture assuming that the slice
is a MachO object. We generalize it and make it return the corresponding instance of ObjectForArch instead;
Test plan: make check-all