Add Buffer object to front end and Memory object to back end.
Implement creation of buffer and sub-buffer objects.
Make cl::Object destructor virtual to support destruction of Buffer.
Cache more Device info for Buffer validation. Add missing and move
existing version checks into validation.

Bug: angleproject:5956
Change-Id: Iea7665be5f6bdd8469e81f5fe4935a9fb0e03436
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2912677Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: John Plate <jplate@google.com>