go/types, types2: report an error for x.sel where x is a built-in

In case of a selector expression x.sel where x is a built-in
we didn't report an error because the type of built-ins is
invalid and we surpress errors on operands of invalid types,
assuming that an error has been reported before.

Add a corresponding check for this case.

Review all places where we call Checker.exprOrType to ensure
(invalid) built-ins are reported.

Adjusted position for index error in types2.

Fixes #51360.

Change-Id: I24693819c729994ab79d31de8fa7bd370b3e8469
Reviewed-on: https://go-review.googlesource.com/c/go/+/388054


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