Turn Driver::CCCIsCXX and CCCIsCPP into a single Mode enum,
and add a new option --driver-mode= to control it explicitly.

The CCCIsCXX and CCCIsCPP flags were non-overlapping, i.e. there
are currently really three modes that Clang can run in: gcc, g++
or cpp, so it makes sense to represent them as an enum.

Having a command line flag to control it helps testing.