(currently spelled with an underscore to indicate its WIP state)
Later commits will handle imported enums correctly and implement the
checks for switch cases.
This is more complex than it could be if ExtensionDecl and NominalTypeDecl
had a common ancestor in the Decl hierarchy, however this is not possible
right now because TypeDecl inherits from ValueDecl.
Fixes <rdar://problem/20981254>.
Swift SVN r28941