Eliminate Program::mLinkResolved boolean

Since mLinkResolved boolean is always kept in sync with the presence
of mLinkingState structure, we can remove the former and replace
access with checks for the latter being nullptr.

Bug: angleproject:4644
Change-Id: Ib385b5c3881946d762ad48e6534c84d15e50f673
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2209319
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>

Author: RafaelCintron