This adds an `appendInterpolation` overload to
`DefaultStringInterpolation` that includes a parameter for providing a
default string when the value to interpolate is `nil`. This allows this
kind of usage:
```swift
let age: Int? = nil
print("Your age is \(age, default: "timeless")")
// Prints "Your age is timeless"
```
The change includes an additional fixit when optional values are
interpolated, with a suggestion to use this `default:` parameter.
In 'protocol', 'Self' is implicitly declared as a generic parameter.
In 'struct' and 'enum', 'Self' is just an alias for the nominal type.
In 'class', 'Self' is a dynamic type and usable only in result types and
inside function bodies.
rdar://problem/61307680
