With the old code model you can use autocompletion while typing a comment. With the Clang code model you cannot. This applies both to regular comments and to doc comments.

I refer to c++ symbols in my comments in lots of different circumstances. For example, I frequently write "separator" comments between groups of functions or methods, e.g.

//
// Public methods for ClassWithAReallyLongNameThatsANuisanceToTypeByHand
//

I also often write inline comments in one method that refer to another, e.g:

// `FooCollection::addFoo(Foo)` guarantees that the array we're using 
// here is always sorted.

Being able to use autocompletion for things like that is really handy. The Clang code model should offer the same behaviour as the old code model in this regard.