glsl: Add ir_var_shader_storage

glsl: Add ir_var_shader_storage

This will be used to identify buffer variables inside shader storage
buffer objects, which are very similar to uniforms except for a few
differences, most important of which is that they are writable.

Since buffer variables are so similar to uniforms, we will almost always
want them to go through the same paths as uniforms.

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