This method previously relied on output from the D3D back-end from
ProgramImpl::link. Split the D3D-specific code into its own method,
and do the linking validation before we call link.

BUG=angleproject:1123

Change-Id: I582671e21af605af9f4451b02fc96ad29c923048
Reviewed-on: https://chromium-review.googlesource.com/293762Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>