[modules-ts] Fold together -x c++ and -x c++-module at -cc1 level.

The driver needs to know whether it's building a module interface or
implementation unit because it affects which outputs it produces and how it
builds the command pipeline. But the frontend doesn't need to know and should
not care: all it needs to know is what action it is being asked to perform on
the input.

(This is in preparation for permitting -emit-obj to be used on a module
interface unit to produce object code without going via a "full" PCM file.)

llvm-svn: 300611