internal/core/adt: fix 2052 for eval v3

Now we finalize the source node for a
for comprehension explicitly, we no longer
need to use "require" to get the initial node.
This avoid some eager evaluation that breaks
2052.

We also relax the added conditions in
CallExpr.evaluate, as it caused a similar
issue with eager evaluation with the
"if" version of a the reducers for Issue
2052.

Issue #2052 (already fixed and closed for v2)

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I016959f07285ba17b555e54234e49386a84dc71f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1206327
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>