[Impeller] Fix a race that can abort the process if the Vulkan context is destroyed while pipeline creation tasks are pending#50883

auto-submit[bot] merged 3 commits intoflutter:mainflutter/engine:mainfrom

jason-simmons:iplr_promise_dtor_racejason-simmons/flutter_engine:iplr_promise_dtor_raceCopy head branch name to clipboard