gl::DrawBuffersVector was used to hold the clear values, but that didn't
have enough space for depth/stencil clear values if MAX draw buffers
where used and cleared.

The added test in this change exposes the vkCmdClearAttachment Qualcomm
bug (previously presumed to affect color clears only) with depth/stencil
buffers, so the workaround is expanded to avoid vkCmdClearAttachment
entirely.

Bug: b/159808300
Change-Id: I27c58d9b534bce0bdd27cc53fc64e139f1363c1a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2455166
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>