[dsymutil] Replace -parse-only option with -dump-debug-map

As the serialized debug map is becoming a first class citizen, a way
to cleanly dump it is required. We used -parse-only combined with
-v for that purpose before, but it dumps a lot of unrelated debug
stuff. Dumping the debug map was the only use of the -parse-only flag
anyway, so replace it with a more useful option.

llvm-svn: 238940