This will allow us to store depth- or stencil-only clear when we defer
clears in FramebufferVk syncing. Currently a refactoring change only.

Bug: angleproject:4517
Change-Id: Ifc9bf8f9ebab993509155cf6cf66db5b11f35163
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2151169Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>