After seen zillions of steel , AL and Ti bikes with dents, cracks i really doubt that stem will fail any day soon and probably never. As for how to fix the dent, you cant fix it unless you want to add some plastic putty used for aluminum body work (I imagine that will work because no idea if there is one for Ti) and paint the stem over it, besides that pretty much there is not much to do.
Just in case, how in the world did you manage to get a dent in there?? 1st time i see something like that.
Good luck.