[5.1] Add support for dynamic replacement of didSet/willSet
The observer in a dynamic replacement of variables with a observer will
provide the dynamic replacement for the original.
var original : Int = 0 {
didSet {
print("original")
}
}
@_dynamicReplacement(for: original)
var replacement : Int {
didSet {
print("replacement")
}
}
rdar://48518788
