[dsymutil] Implement support for universal mach-o object files.

This patch allows llvm-dsymutil to read universal (aka fat) macho object
files and archives. The patch touches nearly everything in the BinaryHolder,
but it is fairly mechinical: the methods that returned MemoryBufferRefs or
ObjectFiles now return a vector of those, and the high-level access function
takes a triple argument to select the architecture.

There is no support yet for handling fat executables and thus no support for
writing fat object files.

llvm-svn: 243096