glsl: Add new atomic_uint built-in GLSL type.

glsl: Add new atomic_uint built-in GLSL type.

v2: Fix GLSL version in which the type became available.  Add
    contains_atomic() convenience method.  Split off atomic counter
    comparison error checking to a separate patch that will handle all
    opaque types.  Include new ir_variable fields for atomic types.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>