all: use cue/token.Pos.IsValid rather than comparing with NoPos
Using the IsValid method is clearer, and a nice side benefit
is that it's a little shorter and sometimes avoids needing an import.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I193c00c8712353486b4a0d88afda00fcec1bfab4
Reviewed-on: https://cue.gerrithub.io/c/cue-lang/cue/+/1227394
Reviewed-by: Matthew Sackman <matthew@cue.works>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>