Auto merge of #104321 - Swatinem:async-gen, r=oli-obk

Avoid `GenFuture` shim when compiling async constructs

Previously, async constructs would be lowered to "normal" generators, with an additional `from_generator` / `GenFuture` shim in between to convert from `Generator` to `Future`.

The compiler will now special-case these generators internally so that async constructs will *directly* implement `Future` without the need to go through the `from_generator` / `GenFuture` shim.

The primary motivation for this change was hiding this implementation detail in stack traces and debuginfo, but it can in theory also help the optimizer as there is less abstractions to see through.

---

Given this demo code:

```rust
pub async fn a(arg: u32) -> Backtrace {
    let bt = b().await;
    let _arg = arg;
    bt
}

pub async fn b() -> Backtrace {
    Backtrace::force_capture()
}
```

I would get the following with the latest stable compiler (on Windows):

```
   4: async_codegen::b::async_fn$0
             at .\src\lib.rs:10
   5: core::future::from_generator::impl$1::poll<enum2$<async_codegen::b::async_fn_env$0> >
             at /rustc/897e37553bba8b42751c67658967889d11ecd120\library\core\src\future\mod.rs:91
   6: async_codegen::a::async_fn$0
             at .\src\lib.rs:4
   7: core::future::from_generator::impl$1::poll<enum2$<async_codegen::a::async_fn_env$0> >
             at /rustc/897e37553bba8b42751c67658967889d11ecd120\library\core\src\future\mod.rs:91
```

whereas now I get a much cleaner stack trace:

```
   3: async_codegen::b::async_fn$0
             at .\src\lib.rs:10
   4: async_codegen::a::async_fn$0
             at .\src\lib.rs:4
```

Author: bors