[dsymutil] Implement support for handling mach-o universal binaries as main input/output.

The DWARF linker isn't touched by this, the implementation links
individual files and merges them together into a fat binary by
calling out to the 'lipo' utility.

The main change is that the MachODebugMapParser can now return
multiple debug maps for a single binary.

The test just verifies that lipo would be invoked correctly, but
doesn't actually generate a binary. This mimics the way clang
tests its external iplatform tools integration.

llvm-svn: 244087