Rollup merge of #59286 - cramertj:async-fn-ret-ty, r=varkor

Refactor async fn return type lowering

async fn now lowers directly to an existential type declaration
rather than reusing the `impl Trait` return type lowering.

As part of this, it lowers all argument-position elided lifetimes
using the in-band-lifetimes machinery, creating fresh parameter
names for each of them, using each lifetime parameter as a generic
argument to the generated existential type.

This doesn't currently successfully allow multiple
argument-position elided lifetimes since `existential type`
doesn't yet support multiple lifetimes where neither outlive
the other:
```rust
existential type Foo<'a, 'b>:; // error: ambiguous lifetime bound in `impl Trait`
fn foo<'a, 'b>(_: &'a u8, _: &'b u8) -> Foo<'a, 'b> { () }
```

This requires a separate fix.

Fix #59001
Fix #58885
Fix #55324
Fix #54974
Progress on #56238

r? @nikomatsakis

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