go/types: combine all type inference in a single function

This is a port of CL 306170 to go/types, adjusted for the different
positioning API.

Some of the error positions in tests had to be adjusted, but I think the
new locations are better.

Change-Id: Ib157fbb47d7483e3c6302bd57f5070bd74602a36
Reviewed-on: https://go-review.googlesource.com/c/go/+/312191


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>