Merge #6921

6921: Higher-ranked trait bounds for where clauses r=flodiebold a=Veykril

There is a slight problem with this which is also noted in a FIXME now but `LifetimeParameters` of these ForLifetime where clauses allocate the lifetimes in the corresponding arena as if they were lifetimes of the item itself and not just the clause they belong to. I wasn't entirely sure what I could do about this but given nothing really uses lifetimes like that currently I figured it might be fine? Open to suggestions for that problem.

Co-authored-by: Lukas Wirth <lukastw97@gmail.com>

crates/hir/src/code_model.rs

@@ -1268,7 +1268,6 @@ impl LifetimeParam {
     }
 }
 
-// FIXME: rename from `ImplDef` to `Impl`
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct Impl {
     pub(crate) id: ImplId,