go/types, types2: more systematic use of Checker.use und useLHS
This CL re-introduces useLHS because we don't want to suppress
correct "declared but not used" errors for variables that only
appear on the LHS of an assignment (using Checker.use would mark
them as used).
This CL also adjusts a couple of places where types2 differed
from go/types (and suppressed valid "declared and not used"
errors). Now those errors are surfaced. Adjusted a handful of
tests accordingly.
Change-Id: Ia555139a05049887aeeec9e5221b1f41432c1a57
Reviewed-on: https://go-review.googlesource.com/c/go/+/478635 Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>