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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243096 91177308-0d34-0410-b5e6-96231b3b80d8