mesa: Report ARB_debug_output for both shader errors and warnings.

mesa: Report ARB_debug_output for both shader errors and warnings.

This ends up reusing the dynamic ID support, so a silly enum gets to go
away.  We don't assign good IDs to different messages yet, but at least
that's tractable now.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>