Auto merge of #115361 - compiler-errors:param, r=cjgillot

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](rust-lang/rust@master...compiler-errors:rust:non-lifetime-binder-where-clauses)).

r? `@cjgillot`

Author: bors