Add HLSL Language Option and Preprocessor

Bringing in HLSL as a language as well as language options for each of
the HLSL language standards.

While the HLSL language is unimplemented, this patch adds the
HLSL-specific preprocessor defines which enables testing of the command
line options through the driver.

Reviewed By: pete, rnk

Differential Revision: https://reviews.llvm.org/D122087