The call to glBindFragDataLocationIndexedEXT was not being properly
forwarded through ANGLE because we assumed that if its location was
set, then its index was set, which is not always the case.
Fixed this, and also added a link status check after linking so the
error will properly propagate in the future.

Bug: chromium:1099763
Change-Id: I28981c04a3f2d6a59c239b35b3f7eb0c7f586804
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2289653Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>