go/types: avoid "declared but not used errors" for invalid variable initializations
This is a partial port of CL 351669 from types2 to go/types; it
only copies the fix for variable usage.
Eventually we may want to use the compiler error messages for assignment
errors everywhere, but that doesn't need to happen now.
Change-Id: I62b024b1b29cc27c29d919de3de44f944f6e2b4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/351670
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>