go/types: use combined type and ordinary args for type inference

This is a port of CL 308372 to go/types. The only meaningful change was
to add TODOs to improve the positioning error messages.

Change-Id: I8314615d0851a59c2b5fd30eb897d581652eacc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/314435


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>