The old code was seriously broken: It assumed that a constant
declaration without a type would always inherit the type of
the previous declaration, but in fact it only inherits the
type of the previous declaration when there's no type and no
constant value.

While fixing this bug, found that the result was not sorted
deterministically in all situations due to a poor choice of
order value (which led to spurious test failures since the
tests assume deterministic outputs). Fixed that as well.

Added new test cases and fixed some old (broken) tests.

Fixes #16153.

Change-Id: I95b480e019b0fd3538638caba02fe651c69e0513
Reviewed-on: https://go-review.googlesource.com/68730Reviewed-by: Alan Donovan <adonovan@google.com>