Vulkan: Simplify SubAllocation data types. We no longer need to bootstrap on the wrapper classes because we use a separate garbage list. This simplifies the code for the allocation tracking considerably. Also we remove a few mutable accessor to fortify our OOP design. (Credit to Jamie since it is splited from his CL) Bug: angleproject:6980 Change-Id: I2025ec4d141531273e824bb586957a38efc30be1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3469715 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>