Also move this type to D3D-only world. It was only used in the D3D
renderer and has specific stuff like register/semantic indexes.

Refactoring patch to clean up further work with Varyings.

BUG=angleproject:1202

Change-Id: I4b1d6899e9eef356efc7d11e9cd6cf88b234aa76
Reviewed-on: https://chromium-review.googlesource.com/311240Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>