swift234

Implement reflection on instances of empty and single-payload enums

This change attempts to introduce the functionality without being too
disruptive. After we branch, I want to consolidate some of the runtime
functions and implement this functionality for multi-payload enums
as well, which requires adding new runtime metadata.

Example:

(swift) enum Color { case Red, Green, Blue(Int) }
(swift) print(Color.Red)
REPL.Color.Red
(swift) print(Color.Blue(5))
REPL.Color.Blue(5)

Implements <rdar://problem/18334936>.

Swift SVN r28430