Auto merge of #140748 - m-ou-se:super-format-args3, r=jdonszelmann

 Allow storing `format_args!()` in variable

Fixes rust-lang/rust#92698

Tracking issue for super let: rust-lang/rust#139076

Tracking issue for format_args: rust-lang/rust#99012

This change allows:

```rust
let name = "world";
let f = format_args!("hello {name}!"); // New: Store format_args!() for later!

println!("{f}");
```

This will need an FCP.

This implementation makes use of `super let`, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have _a_ way of expressing temporary lifetimes like this, since the (stable) `pin!()` macro needs this too. (This was also the motivation for merging rust-lang/rust#139114.)

(This is a second version of rust-lang/rust#139135)

Author: bors