[HLSL] Add DirectXTargetCodeGenInfo and the code paths to convert HLSL types to DirectX target types

[bogner]

Introduce CGHLSLRuntime::convertHLSLSpecificType and TargetInfo::getHLSLType interfaces and implement these in DirectXTargetCodeGenInfo to lower the HLSLResource built in type to DirectX-specific TargetExtTypes.

The interfaces should be similar to the OpenCL interfaces of the same names.

AC:

• The HLSLResource built in type used in HLSL's RWBuffer is converted to a target("dx.TypedBuffer") during clang codegen when targeting DXIL
• It's obvious where and how to add logic for other HLSL buffer and DXIL target types