[AST] Fix the printing of @_objcRuntimeName.

rdar://problem/46646438

Author: mikeash

lib/AST/Attr.cpp

@@ -536,10 +536,10 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
   }
 
   case DAK_ObjCRuntimeName: {
-    Printer.printAttrName("@objc");
+    Printer.printAttrName("@_objcRuntimeName");
     Printer << "(";
     auto *attr = cast<ObjCRuntimeNameAttr>(this);
-    Printer << "\"" << attr->Name << "\"";
+    Printer << attr->Name;
     Printer << ")";
     break;
   }