cmd/compile/internal/types2: refactor untyped conversions

Based on https://golang.org/cl/284256 for go/types.
Brings this code more in line with go/types.

Adjusted various tests to match new error messages which
generally are now better: for assignment errors, instead
of a generic "cannot convert" we now say "cannot use"
followed by a clearer reason as to why not.

Major differences to go/types with respect to the changed
files:

- Some of the new code now returns error codes, but they
  are only used internally for now, and not reported with
  errors.

- go/types does not "convert" untyped nil values to target
  types, but here we do. This is unchanged from how types2
  handled this before this CL.

Change-Id: If45336d7ee679ece100f6d9d9f291a6ea55004d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/302757


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>