Rollup merge of #48909 - RalfJung:type_alias_bounds, r=petrochenkov

Improve lint for type alias bounds

First of all, I learned just today that I was wrong assuming that the bounds in type aliases are entirely ignored: It turns out they are used to resolve associated types in type aliases. So:
```rust
type T1<U: Bound> = U::Assoc; // compiles
type T2<U> = U::Assoc; // fails
type T3<U> = <U as Bound>::Assoc; // "correct" way to write this, maybe?
```
I am sorry for creating this mess.

This PR changes the wording of the lint accordingly. Moreover, since just removing the bound is no longer always a possible fix, I tried to detect cases like `T1` above and show a helpful message to the user:
```
warning: bounds on generic parameters are not enforced in type aliases
  --> $DIR/type-alias-bounds.rs:57:12
   |
LL | type T1<U: Bound> = U::Assoc; //~ WARN not enforced in type aliases
   |            ^^^^^
   |
   = help: the bound will not be checked when the type alias is used, and should be removed
help: use absolute paths (i.e., <T as Trait>::Assoc) to refer to associated types in type aliases
  --> $DIR/type-alias-bounds.rs:57:21
   |
LL | type T1<U: Bound> = U::Assoc; //~ WARN not enforced in type aliases
   |                     ^^^^^^^^
```
I am not sure if I got this entirely right. Ideally, we could provide a suggestion involving the correct trait and type name -- however, while I have access to the HIR in the lint, I do not know how to get access to the resolved name information, like which trait `Assoc` belongs to above. The lint does not even run if that resolution fails, so I assume that information is available *somewhere*...

This is a follow-up for (parts of) https://github.com/rust-lang/rust/pull/48326. Also see https://github.com/rust-lang/rust/issues/21903.

This changes the name of a lint, but that lint was just merged to master yesterday and has never even been on beta.

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