go/types: use TypeList in the Inferred struct

go/types: use TypeList in the Inferred struct

This is for consistency with how we report TArgs elsewhere, and in case
we ever want to share an internal slice with inference reporting.

Change-Id: Ia8b705a155f4f82bd8da8dc2457289810f875f5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/343934
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>