swift234

[ClangImporter] Add convert-inits for enums defined in terms of other enums.

This pattern isn't too uncommon:

typedef CF_ENUM(CFIndex, CFFoo) {
  CFFooBar
};
typedef NS_ENUM(NSInteger, NSFoo) {
  NSFooBar = CFFooBar
};

NSFoo and CFFoo are distinct types, but sometimes you need to convert from
one to the other. Today the only way to do to that is via rawValue, which
is especially unpleasant if the raw types don't match.

This commit introduces a new converting initializer for NSFoo:

  init!(_ value: CFFoo) {
    self.init(rawValue: RawType(value.rawValue))
  }

It's failable by necessity because init(rawValue:) is failable for proper
enums. In practice an audit of our public headers found zero cases where
there are "more" cases in CFFoo than in NSFoo, i.e. cases where the
conversion would fail.

This also applies to option sets and opaque enums, but those use a
non-failable initializer because there's no way to check validity.

rdar://problem/19667625

Swift SVN r25401