go/types: better error message when using comparable in union
This is a port of CL 372674 from types2 to go/types with
minor adjustments for error handling.
For #49602.
Change-Id: I726081325a2ff2d5690d11ddc8a830bbcbd8ab33
Reviewed-on: https://go-review.googlesource.com/c/go/+/372954
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>