[Support] Add MemoryBuffer::getFileSlice()

mach-o supports "fat" files which are a header/table-of-contents followed by a
concatenation of mach-o files built for different architectures. Currently,
MemoryBuffer has no easy way to map a subrange (slice) of a file which lld
will need to select a mach-o slice of a fat file. The new function provides
an easy way to map a slice of a file into a MemoryBuffer. Test case included.