[dsymutil] Add (unimplemented) --flat option

dsymutil should by default generate dSYM bundles which are filesystem
hierarchies containing the debug info and an additional Info.plist.
Currently llvm-dsymutil emits raw binaries containing the debug info.
This is what we call the 'flat mode'. Add a -f/-flat option that is
supposed to enable that flat mode, but don't wire it for now, only
pass it to the tests that will need it to stay functional once we
do bundle generation by default.
This basically makes this commit NFC and removes the noise from the
actual commit that adds support for bundle generation.

llvm-svn: 244269