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

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

This is a clean port of CL 358774 to go/types.

Change-Id: Icba54336de2b8de7c2002d2a44cac856907178c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/360754
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>