ANGLE packs FBO attachments from OpenGL and uses packed attachments to
create VkFramebuffer and renderpass. When we use attachment index into
the attachment array, we must be very careful to use packed index for
vulkan objects. It is easy to make mistakes here and introduce hard to
debug bugs. This CL defines a PackedAttachmentIndex class that uses that
to index into vulkan attachments and pass around APIs so that compiler
would catch the error when wrong index is used. This also introduces
PackedClearValuesArray that stores clear value in packed attachment
index so that it is impossible to mix it with ClearValuesArray that
stores clear value in GL attachment index.

Bug: b/167301719
Change-Id: I68680522c60beeb5096e5211eaef89da28c7097e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2410366
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>