fix: Wrong BoundVar index when lowering impl trait parameter of parent generics
Fixes#17711
From the following test code;
```rust
//- minicore: deref
use core::ops::Deref;
struct Struct<'a, T>(&'a T);
trait Trait {}
impl<'a, T: Deref<Target = impl Trait>> Struct<'a, T> {
fn foo(&self) -> &Self { self }
fn bar(&self) {
let _ = self.foo();
}
}
```
when we call `register_obligations_for_call` for `let _ = self.foo();`,
07659783fd/crates/hir-ty/src/infer/expr.rs (L1939-L1952)
we are querying `generic_predicates` and it has `T: Deref<Target = impl Trait>` predicate from the parent `impl Struct`;
07659783fd/crates/hir-ty/src/lower.rs (L375-L399)
but as we can see above, lowering `TypeRef = impl Trait` doesn't take into account the parent generic parameters, so the `BoundVar` index here is `0`, as `fn foo` has no generic args other than parent's,
But this `BoundVar` is pointing at `'a` in `<'a, T: Deref<Target = impl Trait>>`.
So, in the first code reference `register_obligations_for_call`'s L:1948 - `.substitute(Interner, parameters)`, we are substituting `'a` with `Ty`, not `Lifetime` and this makes panic inside the chalk.
This PR fixes this wrong `BoundVar` index in such cases
469b06214a