Remove void from the list of opaque types.

`void` behaves differently from opaque types in several situations, such
as function return types, and errors involving `void` should not call it
an opaque type. I've fixed all the places where we relied on `isOpaque`
to return catch void types.

Change-Id: I359c00f836be6e56cb0373beff60cf2ff830f77b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/472451
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed