go/types, cmd/compile: fix composite literal structural typing

For a composite literal expression like []T{{f: 1}}, we allow T to be
a pointer to struct type, so it's consistent to allow T to also be a
type parameter whose structural type is a pointer to struct type.

Fixes #50833.

Change-Id: Ib0781ec4a4f327c875ea25b97740ff2c0c86b916
Reviewed-on: https://go-review.googlesource.com/c/go/+/381075


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>