go/types, types2: simplify Checker.typeAssertion, use same code in both type checkers

- Remove the xtyp argument from the Checker.typeAssertion parameter
  list; it was confusing and not needed. Adjusted call sites.

- Simplify logic in Checker.typeAssertion.

- Use the same code in both types2 and go/types, specifically
  use the same error positions.

- Adjust error messages as needed.

This removes another subtle discrepancy between types2 and go/types.
The go/types error messages don't have the have/want appendix for
the affected error messages yet because we don't use case folding
in lookups yet.

Change-Id: Id39f5c473da36c9baad60082f85cf1f34dc26c50
Reviewed-on: https://go-review.googlesource.com/c/go/+/383014


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