go/types: reduce number of delayed functions

This is a port of CL 348018 to go/types. It differs from that CL due to
the way that field lists are represented in go/ast.

Change-Id: Ib5a0243b44d0bf9e95d039f624c668f8c329f8fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/348691


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>