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>

src/mesa/main/shader_query.cpp

@@ -86,6 +86,15 @@ is_active_attrib(const ir_variable *var)
    case ir_var_shader_in:
       return var->data.location != -1;
 
+   case ir_var_system_value:
+      /* From GL 4.3 core spec, section 11.1.1 (Vertex Attributes):
+       * "For GetActiveAttrib, all active vertex shader input variables
+       * are enumerated, including the special built-in inputs gl_VertexID
+       * and gl_InstanceID."
+       */
+      return !strcmp(var->name, "gl_VertexID") ||
+             !strcmp(var->name, "gl_InstanceID");
+
    default:
       return false;
    }