Whenever a struct type is decorated Block or BufferBlock we turn that
into a GLSL_TYPE_INTERFACE. Since these decorations can end up random
places, we should allow them for constants.
Closes: #3252
Fixes: 9d0ae777dd "spirv: Use interface type for block and buffer..."
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5855>