[react-native] Some iosUtils code paths are possibly not being used

[BethanyBerkowitz]

The iosUtils code came from Andrey's original code and I took pains not to mess with how it works. However, in my testing, I've only found backtraces generated using framesFromIOSCallStack. I've never seen an error reported with primaryBackTraceSource = 'ReactNativeComponentStack' or primaryBackTraceSource = 'ReactNativeIOSStackTrace'.

See #1090 (comment) for more detail.

Two main concerns here:

1. Check if the ReactNativeComponentStack code path is being triggered in dev and how. If so, add a unit test for that, a code comment explaining how it works, and ideally a way to trigger this type of error in at least one of the example projects.
2. In prod, we are not currently passing to the built-in handler, so errors are only going through the JS handler. We did this to avoid duplicate error reports, but we may be getting less detail than we would otherwise. Check if we are losing detail this way... if so what should we do about that?