[HLSL] add -fcgl option flag.

fcgl option will make compilation stop after clang codeGen and output the llvm ir.
It is added to check clang codeGen output for HLSL.

It will be translated into -S -emit-llvm and -disable-llvm-passes.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D124983