Vulkan descriptor shifting

[Amer Koleci]

Is your feature request related to a problem? Please describe.
Its not a problem but probably good improvements when porting shaders to vulkan backend.

Describe the solution you'd like
Add the missing -fvk-{b|s|t|u}-shift options (https://github.com/Microsoft/DirectXShaderCompiler/blob/master/docs/SPIR-V.rst).

Describe alternatives you've considered
None

Additional context
None

[Tibor Klajnscek]

I second this.

For now, I've just added a new member 'compilerArgs' to SourceDesc so they get passed to DXC. If this is enough, I can do a PR, otherwise it can be formally added to Options I guess, but it might be overkill.

Edit: On second thought, I'll just add it to Options as that's the way it's designed to be :)