The LinkedVarying structure is a D3D-specific type, and the GL
back-end doesn't need the extra info. Isolate this into the D3D
back-end so we can clean up the Impl inteface.

BUG=angleproject:1123

Change-Id: I76d77ac505876d865e3e02f47acbfd6665a9507e
Reviewed-on: https://chromium-review.googlesource.com/293764Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>