[HLSL] Pass flags to cc1 based on language

Having the flags only pass through if you're using the dxc-driver means
that the clang driver doesn't work for HLSL, which is undesirable. This
change switches to instead passing flags based on the language mode
similar to how OpenCL does it. This allows the clang driver to be used
for HLSL source files as well.

Reviewed By: python3kgae

Differential Revision: https://reviews.llvm.org/D133958