[stdlib][Foundation] Avoid coexistence conflicts with JSONEncoder/Decoder and _stdlib_AtomicInt.
[stdlib][Foundation] Avoid coexistence conflicts with JSONEncoder/Decoder and _stdlib_AtomicInt.

JSONEcoder/Decoder get another underscore in their ObjC name, as the one with a single underscore conflicts with older Swifts that had _JSONEcoder/Decoder as internal rather than fileprivate.

_JSONEncoder/Decoder get another underscore in their Swift name to avoid conflicting with the ObjC name from the ones in older overlays.
apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation