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

 Allow storing `format_args!()` in variable

Fixes https://github.com/rust-lang/rust/issues/92698

Tracking issue for super let: https://github.com/rust-lang/rust/issues/139076

Tracking issue for format_args: https://github.com/rust-lang/rust/issues/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 https://github.com/rust-lang/rust/pull/139114.)

(This is a second version of https://github.com/rust-lang/rust/pull/139135)

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