In the situation when we try to clear the depth OR the stencil
buffer but we have a packed depth/stencil attachment, we were still
clearing both of them since Vulkan doesn't allow setting just the depth
or the stencil image aspect bit on the clear call. The workaround is to
use the vkCmdClearAttachments command that allows us to clear one or
the other easily.

Bug: angleproject:2443

Change-Id: I6814e1927c019d9ec9255d79c6bc7b913a8c99e9
Reviewed-on: https://chromium-review.googlesource.com/997752
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>