Don't accept -std= values that would switch us to a different source language.

We already prohibited this in most cases (in r130710), but had some bugs in our
enforcement of this rule. Specifically, this prevents the following
combinations:

 * -x c -std=clN.M, which would previously effectively act as if -x cl were
   used, despite the input being a C source file. (-x cl -std=cNN continues
   to be disallowed.)

 * -x c++ -std=cuda, which would previously select C++98 + CUDA, despite that
   not being a C++ standard. (-x cuda -std=c++NN is still permitted, and
   selects CUDA with the given C++ standard as its base language.
   -x cuda -std=cuda is still supported with the meaning of CUDA + C++98.)

 * -x renderscript -std=c++NN, which would previously form a hybrid "C++ with
   RenderScript extensions" language. We could support such a thing, but
   shouldn't do so by accident.

llvm-svn: 301497