Instead of storing varying information in the shader, use a temporary
set when linking a D3D program. This also means we won't have to
modify information in the Shader object when linking a D3D program.

This completes the refactoring for PackedVaryings.

Re-land with fix for missing init of PackedVarying::vertexOnly.

BUG=angleproject:1123

Change-Id: If110809c3817d88b0370ac575d739d7385b067d9
Reviewed-on: https://chromium-review.googlesource.com/296731Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>