Vulkan does not support separate depth and stencil attachments.
Adding this check to checkStatus skips the relevant tests to that.

There was also a bug in FramebufferVk::clear where we were not
checking the number of bits for depth/stencil attachments before
setting the aspects for clearing.

Bug: angleproject:2597
Change-Id: Iabe9f77a51fc7aca08b8faf3cecc3df9f99a7f1e
Reviewed-on: https://chromium-review.googlesource.com/1107847
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>