[JSONSerialization] Always use reference numeric types to match Darwin behavior

[xwu]

This PR removes the undocumented internal serialization option useReferenceNumericTypes and aligns behavior with Darwin Foundation. (Note: there remains a divergence between platforms when it comes to 128-bit integers.)

The parsing logic is simplified and tests are expanded to ensure that "[10]" is deserialized to a value that can be cast to Int or Double.

[xwu]

/cc @johnno1962

[xwu]

@swift-ci please test

[xwu]

@johnno1962 Whoops, looks like we did the same thing. Sorry!

[johnno1962]

Pinch my PR will you 😀!. Yours is better, run with it. Don’t forget to cherry pick into swift-4.2-branch

[parkera]

Looks good to me. I'm assuming we can do this now because of the bridging improvements?

[johnno1962]

Yes, this is following through on #1443

[itaiferber]

The changes here specifically LGTM. I wonder if now that we can finally make use of bridging whether we should move off Int as the primary integer type to Int64 and add support for UInt64 to get the full range of integer values as we do on Darwin.

[xwu]

@itaiferber Certainly a good idea; I'll keep this PR limited to the changes here, and that can be a follow-on improvement. As I mention, we still have some discrepancies in that department with respect to Darwin support for 128-bit integers.

[itaiferber]

@swift-ci Please test and merge

[itaiferber]

@xwu Now that this is merged, please also cherry pick into swift-4.2-branch