spirv: Use interface type for block and buffer block

spirv: Use interface type for block and buffer block

Also handle GLSL_TYPE_INTERFACE the same way we do GLSL_TYPE_STRUCT in
various places.  Motivated by ARB_gl_spirv work, that will take
advantage of the interface types when handling NIR coming from SPIR-V.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>