Driver: support detecting driver mode when clang has a version suffix without dash (PR21094)

Clang would previously not get into C++ mode when invoked as 'clang++3.6'
(though clang++-3.6 would work).

I found the previous loop logic in this function confusing; hopefully this
makes it a little clearer.

Differential Revision: http://reviews.llvm.org/D5833