Improve error message for "return type cannot contain a projection or Self..."

[tmandry]

Example (playground):

error: `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
 --> src/lib.rs:4:33
  |
4 |     async fn new(i: &'a i32) -> Self {
  |                                 ^^^^

• Add a hint: try spelling out the type instead
• Add an error code with an entry in the error index explaining that this is a limitation of the compiler, tracked by Inherent async fn returning Self treats type's lifetime parameters as 'static #61949

[kornelski]

I've ran into this case, which doesn't have a good diagnostic either:

struct Foo<'a>(&'a ());

impl<'a> Foo<'a> {
    async fn named(arg: &'a ()) -> Foo<'a> {
        Self(arg)
    }
    
    async fn selfie(arg: &'a ()) -> Self {
        Self(arg)
    }
}