Diagnose redeclarations of Objective-C methods.

@objc methods, initializers, deinitializers, properties, and
subscripts all produce Objective-C methods. Diagnose cases where two
such entities (which may be of different kinds) produce the same
Objective-C method in the same class.

As a special exception, one can have an Objective-C method in an
extension that conflicts with an Objective-C method in the original
class definition, so long as the original class definition is from a
different model. This reflects the reality in Objective-C that the
category definition wins over the original definition, and is used in
at least one overlay (SpriteKit).

This is the first part of rdar://problem/18391046; the second part
involves checking that overrides are sane.

Swift SVN r23147

include/swift/AST/ASTContext.h

@@ -642,6 +642,19 @@ public:
   /// \brief Returns memory used exclusively by constraint solver.
   size_t getSolverMemory() const;
 
+  /// Note that there is a conflict between different definitions that
+  /// produce the same Objective-C method.
+  void recordObjCMethodConflict(ClassDecl *classDecl, ObjCSelector selector,
+                                bool isInstance);
+
+  /// Diagnose all conflicts between members that have the same
+  /// Objective-C selector in the same class.
+  ///
+  /// \param sf The source file for which we are diagnosing conflicts.
+  ///
+  /// \returns true if there were any conflicts diagnosed.
+  bool diagnoseObjCMethodConflicts(SourceFile &sf);
+
 private:
   friend class Decl;
   Optional<RawComment> getRawComment(const Decl *D);