cmd/compile/internal/types2: pass pos argument to Checker.overflow

cmd/compile/internal/types2: pass pos argument to Checker.overflow

This matches the go/types version of Checker.overflow.
Preparation for generating this function (and others)
for go/types.

Change-Id: I84117203247011bd2e96c9cf53fd5a443e528bbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/493558
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>