Don't manually compute param indices when adding implicit `Sized` and `ConstParamHasTy`
We can just use resolve_bound_vars to compute these indices faithfully.
This also makes the code easier to support where clauses on `non_lifetime_binders` (like `where for<T: Debug> ...`, etc, which I've got a [WIP implementation of](https://github.com/rust-lang/rust/compare/master...compiler-errors:rust:non-lifetime-binder-where-clauses)).
r? `@cjgillot`