JoinableTaskContext.IsOnMainThread should satisfy VSTHRD010

Is your feature request related to a problem? Please describe.

Previously commented in another issue: https://github.com/microsoft/vs-threading/issues/272#issuecomment-387459857

I have a method I want to be free-threaded, but I have dependencies which are not. How I implement my method depends whether it's running on the main thread, or a background thread. Copying the example from the other issue:

public void NotAffinitized() {
  if (IsOnMainThread()) {
    // This incorrectly marks the enclosing method as affinitized, which then propagates
    SomeAffinitizedMethod();
  } else {
    SomeNonAffinitizedMethod();
  }
}

Describe the solution you'd like

No VSTHRD010 within the if block.

Describe alternatives you've considered

An await jtf.SwitchToMainThreadAsync(alwaysYield: false) does suppress the warning, but feels inefficient. Even if it's fast, it's still extra work that isn't necessary for program correctness, just to satisfy the analyzer.

Additional context

Start by reading JoinableTaskContext.IsOnMainThread and the VSTHRD010 analyzer behavior described in the issue, then inspect the existing analyzer tests. The work is done when checking IsOnMainThread no longer produces VSTHRD010 for the following main-thread-dependent call while other diagnostics remain correct.