Sema: Move semantic constraints on super/self.init out of the parser.

Instead of forcing full application of '{super,self}.init' in the parser, and installing the RebindSelf semantic expr node early, make these constraints to Sema-time checks, and parse '<expr>.init' as a regular postfix production. This is a better separation of concerns, and also opens the door to supporting 'metatype.init()' in more general expression contexts (though that part still needs some follow-up sema work).

Swift SVN r29343

lib/Sema/CodeSynthesis.cpp

@@ -2263,7 +2263,6 @@ swift::createDesignatedInitOverride(TypeChecker &tc,
     superCall = new (ctx) TryExpr(SourceLoc(), superCall, Type(),
                                   /*Implicit=*/true);
   }
-  superCall = new (ctx) RebindSelfInConstructorExpr(superCall, selfDecl);
   ctor->setBody(BraceStmt::create(tc.Context, SourceLoc(),
                                   ASTNode(superCall),
                                   SourceLoc(),