cmd/compile/internal/gc: add comment and test for #15550

cmd/compile/internal/gc: add comment and test for #15550

When switching to the new parser, I changed cmd/compile to handle iota
per an intuitive interpretation of how nested constant declarations
should work (which also matches go/types).

Note: if we end up deciding that the current spec wording is
intentional (i.e., confirming gccgo's current behavior), the test will
need to be updated to expect 4 instead of 1.

Updates #15550.

Change-Id: I441f5f13209f172b73ef75031f2a9daa5e985277
Reviewed-on: https://go-review.googlesource.com/36122
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>