cue: rename RequireConcrete to Concrete
This is more consistent and also allows it to be used
for Syntax to select less evaluation.
Change-Id: I0103c7edd3d523c33df0d0575299b02e2d38adfe
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/1922
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
authored by