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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238940 91177308-0d34-0410-b5e6-96231b3b80d8