[dev.typeparams] go/types, cmd/compile/internal/types2: fix incorrect string(int...

[dev.typeparams] go/types, cmd/compile/internal/types2: fix incorrect string(int) conversion (regression)

This is a 1:1 port of the go/types changes in
https://golang.org/cl/272666 (master branch).

Updates #42790.

Change-Id: I5da372961df48129b25777ed705b84d7201393ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/272669
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>