In preparation for support Program Pipeline Objects, the new function
Context::getActiveLinkedProgram() is being created that will handle
returning the correct Program whether there is currently a Program or
PPO active.

This initial implementation doesn't actually support PPOs yet; that
work is done in a follow-on CL.

Bug: angleproject:3570
Test: CQ
Change-Id: I534261738a55b2f671ca152c2ef9a1e81517a99c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2135931
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>