[stdlib] Dictionary.mapValues with key context

[tayloraswift]

implementation for swiftlang/swift-evolution#3060

it seems this requires @available annotations, i am not sure what the correct attribute to apply here is

+Func Dictionary._Variant.mapValues(_:) is a new API without '@available'
+Func Dictionary.mapValues(_:) is a new API without '@available'
+Func _NativeDictionary.mapValues(_:) is a new API without '@available'
+Func __CocoaDictionary.mapValues(_:) is a new API without '@available'

[tayloraswift]

@swift-ci Please test