Clear temp var for captured var expr to permit GC

[som-snytt]

Fixes #14198

[som-snytt]

That was just non-trivial enough to learn something. Running the suite locally shows errors, so converting to draft while I chase those.

Still to answer, what does a tree inherit by virtue of cpy (position?), and why transformFollowing is called twice.

Update is that resetting the temporary requires null.asInstanceOf[T]. Checker complained where T is Thing[], but I haven't clarified all the post-erasure aspects.

That was in MegaPhase where var nx: Array[MiniPhase] is set by range.foreach { nx = ??? }. I think in Scala 2 the closure is eliminated, because there was a comment

// OPT: hand rolled version of sum

but the var sum is captured by range.foreach, as spuriously mentioned by -Wperformance. Obviously this level of "code switching" (so to speak) could prove quite stressful.

Hardest part still is trying to figure out how to run tests under dotty.

[anatoliykmetyuk]

@som-snytt what's the status on this one? Is it review-ready?

[som-snytt]

Possibly the extra nullifier could be emitted only selectively.

[som-snytt]

Documented the change, aligned the stars, and omit nullify for x = null.

[smarter]

Thank you!