Stack traces with async variants

The stack trace metadata would likely only have information about the thunk, not about the the calling method.

So doing `DiagnosticMethodInfo.Create(new StackFrame())` in a runtime async method will likely return no information. We need to swap IsAsyncCall bodies for thunks when emitting stack trace information.

Also figure out what CoreCLR does for `new StackFrame(1)` (i.e. asking for the thunk).

From @davidwr This is also an issue for the coreclr interpreter, as it needs a helper function, which is defined in C#, and there is no way to disable that either.