swift234

[ClangImporter] Import enum case aliases using static let properties.

I don't want to call this complete until
a) we handle using alias names in switch statements (right now they're
   straight-up rejected, not just non-complete), and
b) we prefer non-deprecated names over deprecated ones to be the "real"
   enum cases.
but this is a good start, and fixes them showing up poorly in the SDK
analyzer.

rdar://problem/18662118

Swift SVN r27130