glsl: Fix typo in interface block comment

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: Extract is_active_attrib() in shaderapi

The rules are about to get a bit more complex to account for
gl_InstanceID and gl_VertexID, which are system values.

Extracting this first avoids introducing duplication.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
mesa: Consider gl_VertexID and gl_InstanceID active attribs

Fixes piglit's spec/gl-3.2/get-active-attrib-returns-all-inputs.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>