[dev.typeparams] go/types: delay interface check for type bounds

[dev.typeparams] go/types: delay interface check for type bounds

This is a port of CL 331690 to go/types. It diverges from that CL due to
the different representation of Fields in the AST.

Change-Id: I3ae9ac3a0172dc58ac748f28772d87b00db0732a
Reviewed-on: https://go-review.googlesource.com/c/go/+/335034
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>