WIP: HLSL: hlsl register class iomapping

Adds --hlsl-iomap option to perform IO mapping in HLSL register space.

--shift-cbuffer-binding is now a synonym for --shift-ubo-binding.

The idea way to do this seems to be passing in a dedicated IO resolver, but
that would require more intrusive restructuring, so maybe best for its
own PR.

The TDefaultHlslIoResolver class and the former TDefaultIoResolver class
share quite a bit of mechanism in a common base class.

TODO: tbuffers are landing in the wrong register class, which needs some
investigation.  They're either wrong upstream, or the detection in the
resolver is wrong.