go/types: clean up asT converters (step 1 of 2)

This is a port of CL 358597 to go/types. A comment was missing in the
base of applyTypeFunc, which had been there since the initial check-in
of types2; somehow it was not in go/types.

Change-Id: If08efd92d782dd3099b26254ae6e311c6cea8c3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/360477


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>