NeXT: Clean up dispatch method policy selection.

 - Replace -cc1 level -fobjc-legacy-dispatch with -fobjc-dispatch-method={legacy,non-legacy,mixed}.

 - Lift "mixed" vs "non-mixed" policy choice up to driver level, instead of being buried in CGObjCMac.cpp.

 - No intended functionality change.

llvm-svn: 102255