Teach llvm-size to know about Mach-O universal files (aka fat files) and
fat files containing archives.

Also fix a bug in MachOUniversalBinary::ObjectForArch::ObjectForArch()
where it needed a >= when comparing the Index with the number of
objects in a fat file.  As the index starts at 0.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211230 91177308-0d34-0410-b5e6-96231b3b80d8