go/types, types2: don't crash if comp. literal element type has no core type

Instead, report a suitable error.

Fixes #51335.

Change-Id: Ifce90cb7487b1e99c6b4221c0d43bacc0c39dca8
Reviewed-on: https://go-review.googlesource.com/c/go/+/387676


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