This change pulls some code from the DirectX backend into a new
LLVMFrontendHLSL library to share utility data structures between the
HLSL code generation in Clang and the backend in LLVM.
This is a small refactoring as a first start to get code into the
right structure and get the library built and dependencies correct.
Fixes#58000 (https://github.com/llvm/llvm-project/issues/58000)
Reviewed By: python3kgae
Differential Revision: https://reviews.llvm.org/D135110
Chris Bieneman