Also declare GroupScope as NO_DISCARD.

Bug: angleproject:6016
Change-Id: I738f137727d52a0930cd8c400fbc05f1cc8cfdad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2943927
Commit-Queue: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>