swift234

[FixCode] Add a fixit to help users migrate to Swift 3 name convention. (#4058)

* [FixCode] Add a fixit to help users migrate to Swift 3 name convention of enum cases. rdar://26887735

When users' referring to a enum case with a wrong name and we can find a correct enum case whose name
differs from the wrong name only in capitalization, we replace the wrong name with the correct one.

* Addressing Argyrios' code review comments. NFC

* [test] Update existing test.

* Grammatical polish suggested by @CodaFi.