[dev.typeparams] go/token, go/scanner: add the "~" operator

[dev.typeparams] go/token, go/scanner: add the "~" operator

This is an approximate port of CL 307370 to go/token and go/scanner.

Change-Id: I5b789408f825f7e39f569322cb67802117b9d734
Reviewed-on: https://go-review.googlesource.com/c/go/+/324992
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>