glsl: Add an origin pointer in the function signature object.

glsl: Add an origin pointer in the function signature object.

This points to the object with the function body, allowing us to map
from a built-in prototype to the actual body with IR code to execute.

Signed-off-by: Olivier Galibert <galibert@pobox.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>