glsl/types: Add a bare "sampler" type

glsl/types: Add a bare "sampler" type

This is to be used by SPIR-V for representing a sampler that isn't attached
to any particular image.  In SPIR-V, all of the interesting bits such as
dimensionality, sampled type, etc. come from the image, the bare "sampler"
type simply uses a sampled type of VOID and 0 values for the rest.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>