cmd/compile/internal/types2: set tparamsIsIface to true
This CL enables the mode in which the underlying type of
type parameters is the underlying type of their constraints.
Change-Id: Id3471578dab098695dbd1e0429356ebcc9c5e224
Reviewed-on: https://go-review.googlesource.com/c/go/+/363155
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>