Auto-attach to service hub process on debug

[jmarolf]

Set the environment variable SERVICEHUBDEBUGHOSTONSTARTUP to All. This will pop up a debug dialog whenever a ServiceHub Host process starts running. You will then be able to attach a debugger to that process. In more complex scenarios, this same environment variable can be set with the name of the specific ServiceHub Host process you would like debug dialogs to be shown for. ex. ServiceHub.Host.CLR.exe.

I think we just want to set this value in environment we launch VS in.

[sharwell]

In the current form, this will impact Ctrl+F5 scenarios (start without debugging). There are two changes needed:

1. This should only impact F5 scenarios (start debugging)
2. This should automatically attach the managed debugger from the current process, as opposed to calling Debugger.Launch and forcing the user to complete the process

An example of (2) can be seen in our integration tests.

roslyn/src/VisualStudio/IntegrationTest/TestUtilities/VisualStudioInstance.cs

Lines 104 to 115 in 075251e

	if (System.Diagnostics.Debugger.IsAttached)
	{
	// If a Visual Studio debugger is attached to the test process, attach it to the instance running
	// integration tests as well.
	var debuggerHostDte = GetDebuggerHostDte();
	var targetProcessId = Process.GetCurrentProcess().Id;
	var localProcess = debuggerHostDte?.Debugger.LocalProcesses.OfType<EnvDTE80.Process2>().FirstOrDefault(p => p.ProcessID == hostProcess.Id);
	if (localProcess != null)
	{
	localProcess.Attach2("Managed");
	}
	}

[jmarolf]

@sharwell I was also thinking of moving this to a separate profile such as Visual Studio Extension (with ServiceHub)

[jmarolf]

This should automatically attach the managed debugger from the current process, as opposed to calling Debugger.Launch and forcing the user to complete the process

This is not possible with service hub today. If we fully owned the process we could do this

[sharwell]

I was also thinking of moving this to a separate profile such as Visual Studio Extension (with ServiceHub)

Sure, this would be totally fine. VS with ServiceHub debugging might be more clear.