glsl: Add ir_variable::interface_type field

glsl: Add ir_variable::interface_type field

For variables that are in an interface block or are an instance of an
interface block, this is the GLSL_TYPE_INTERFACE type for that block.

Convert the ir_variable::is_in_uniform_block method added in the
previous commit to use this field instead of ir_variable::uniform_block.

v2: Fix the place-holder comment on ir_variable::interface_type.
Suggested by Paul Berry.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>