This also isolates headers for D3D9 / D3D11 from each other.

Bug: angleproject:3943
Change-Id: I04edbe7db68461ae4fc78ac7f9c22451debcb768
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2405807
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>