Instead of storing the entire LinkedResources struct, we can keep it
only for the duration of the linking calls. Refactoring change only.
It sets the stage for more refactoring. This change also switches the
link call to use LinkingState's ProgramLinkedResources directly to
avoid the need to copy the varying packing or use a pointer.

Bug: angleproject:4514
Bug: angleproject:5496
Change-Id: Iefea3c16a33213dc338cc54efaa7c3064ea6ae08
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2601403Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>