Review fire-and-forget asynchrony

As highlighted by @jmarolf in https://github.com/dotnet/project-system/pull/4529#discussion_r252798620, we have some async operations which fire-and-forget off into the ether. We should review whether these are subject to potential problems in case of project closure or task faults.

- There are two discarded tasks in `DependenciesProjectTreeProvider`
~- We have two calls to `TplExtensions.Forget` (which does nothing with the task at all)~
~- We have one call to `VsTaskLibraryHelper.FileAndForget` (which handles faults and reports them via telemetry)~
- Possibly more cases elsewhere