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/+/2209319Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>