go/types, types2: slightly tighter checks in Checker.use

go/types, types2: slightly tighter checks in Checker.use

Checker.use is called to check expressions and "use" variables
in case of an error. Use Checker.exprOrType instead of just
rawExpr.

Change-Id: I4da6fa51ef3b0c9b07c453494452836caced9b1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/479897
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>