Review fire-and-forget asynchrony

[drewnoakes]

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

[davkean]

Fire and forgets were banned here: https://github.com/dotnet/project-system/pull/4554. Discarded tasks were not reviewed, but should call IProjectFaultHandlerService.RegisterFaultHandler2 (or whatever the names ends up in the previous PR).