go/types: ensure that error code values do not change in 1.17

Over this cycle some error code values have changed due to codes being
added/removed. This is probably OK to do once more before we export
error codes in a later Go version, but for now let's keep them stable.

Move things around to correct the changes, and update comments in
errorcodes.go to make it clearer that new codes should be added at the
end.

Change-Id: Id32827ef1a72cfd876ccc039da11d0a1be7470e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/314830


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>