This CL adds a new D3D11on12 device option which runs the D3D11
API on top of D3D12. This is done to aid in preliminary
investigations into the feasibility of creating a full D3D12
backend implementation.

Bug: angleproject:3919
Change-Id: I0ad4250eb3c93b0b74274c904aac74f03753c7ad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1814404
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>