Implement Clang importer support for finding Objective-C methods by selector.

This functionality doesn’t really change what we accept right now, because we eagerly import all of the methods of a class when we do *any* kind of lookup into the class. However, when we manage to stop doing that, this operation will become more important.

Swift SVN r23289

include/swift/AST/ModuleLoader.h

@@ -103,7 +103,7 @@ public:
                  ObjCSelector selector,
                  bool isInstanceMethod,
                  unsigned previousGeneration,
-                 llvm::TinyPtrVector<AbstractFunctionDecl *> &methods) { }
+                 llvm::TinyPtrVector<AbstractFunctionDecl *> &methods) = 0;
 
   /// \brief Verify all modules loaded by this loader.
   virtual void verifyAllModules() { }