glsl: Set the precisions of builtin function arguments and returns.

glsl: Set the precisions of builtin function arguments and returns.

These have precision qualifiers defined in the spec, in which case we
should emit them them while generating builtin signatures and code.  We've
been special-casing them in GLSL lower_precision, but now we can just rely
on the precision qualifier of the builtin if non-NONE.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21666>