Use ClassDecl::ForeignKind to model Clang's objc_runtime_visible.

We're now correctly checking for inheritance, adding @objc methods,
and adding @objc protocols for both CF types and objc_runtime_visible
classes (those without visible symbols). The latter is used for some
of the types in Dispatch, which has exposed some of the classes that
were considered implementation details on past OSs.

We still don't properly implement using 'as?' to check conformance to
a Swift protocol for a CF or objc_runtime_visible type, but we can do
that later.

rdar://problem/26850367

include/swift/AST/Decl.h

@@ -3315,10 +3315,6 @@ public:
   /// the Objective-C runtime.
   StringRef getObjCRuntimeName(llvm::SmallVectorImpl<char> &buffer) const;
 
-  /// Determine whether the class is only visible to the Objective-C runtime
-  /// and not to the linker.
-  bool isOnlyObjCRuntimeVisible() const;
-
   /// Returns the appropriate kind of entry point to generate for this class,
   /// based on its attributes.
   ///