go/types: report cause for failing const conversions
This is a port of CL 360795 to go/types. Error messages were adjusted
accordingly, with a TODO to fix the discrepancy.
Change-Id: Ifd7d8248fa11a31fde391021f3c5f1840877892f
Reviewed-on: https://go-review.googlesource.com/c/go/+/360937
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>