go/types: clarify is/underIs semantics and implementation
This is a port of CL 358594 to go/types. Some code in conversions.go had
to be trivially reorganized to align with types2 -- I'm not sure how
go/types diverged from the base.
Change-Id: I40ce247bbb3b9d0e87ce88c50e440c12774c0745
Reviewed-on: https://go-review.googlesource.com/c/go/+/360475
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@golang.org>