[generator|tests] Leverage Apple's Main Thread Checker to get correct [ThreadSafe] checks

### Description

We currently try to guess what is thread safe based on documentation, experience and reports like [5235](https://github.com/xamarin/xamarin-macios/issues/5235), [5234](https://github.com/xamarin/xamarin-macios/issues/5234), [5233](https://github.com/xamarin/xamarin-macios/issues/5233), etc. In Xcode 9, Apple [introduced Main Thread Checker](https://developer.apple.com/documentation/code_diagnostics/main_thread_checker?language=objc) we should find a way to leverage it in our advantage maybe as part of one of our test suites (intro) or as a brand new test.

We might need to adjust the generator once we find a way to do the above.