Moved vk::CommandBuffer and vk::SecondaryCommandBuffer to vk::priv:: and
aliased vk::CommandBuffer to one or the other.  This allows the rest of
the classes to continue seeing vk::CommandBuffer as they used to do.
Used a special alias for the primary command buffer that gets submitted
(vk::PrimaryCommandBuffer).

Bug: angleproject:3136
Change-Id: I61236fd182230991db7395d05e3da3be5e3f45be
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1534456Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>