cmd/compile/internal/types2: fix incorrect result type of Checker.index

While at it, add missing "invalid argument: " prefix
to a couple of local error messages, for consistency.

For #45667.

Change-Id: I814800b2f3f3750583e335c98a3f8e27030a9daa
Reviewed-on: https://go-review.googlesource.com/c/go/+/312309


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