Add HLSL validation

Would be useful to use glslang/Vulkan to develop HLSL without using fxc. This requires implementing validation of HLSL input in glslang.

Start by examining glslang’s existing partial HLSL front end and how glslang/Vulkan validation is currently invoked. Compare the requested validation behavior with fxc, then define the required scope and tests; done means HLSL input can be validated through glslang without using fxc.

Written by the indexing model from the issue text.