Auto merge of #82447 - Amanieu:legacy_const_generics, r=oli-obk

Add #[rustc_legacy_const_generics]

This is the first step towards removing `#[rustc_args_required_const]`: a new attribute is added which rewrites function calls of the form `func(a, b, c)` to `func::<{b}>(a, c)`. This allows previously stabilized functions in `stdarch` which use `rustc_args_required_const` to use const generics instead.

This new attribute is not intended to ever be stabilized, it is only intended for use in `stdarch` as a replacement for `#[rustc_args_required_const]`.

```rust
#[rustc_legacy_const_generics(1)]
pub fn foo<const Y: usize>(x: usize, z: usize) -> [usize; 3] {
    [x, Y, z]
}

fn main() {
    assert_eq!(foo(0 + 0, 1 + 1, 2 + 2), [0, 2, 4]);
    assert_eq!(foo::<{1 + 1}>(0 + 0, 2 + 2), [0, 2, 4]);
}
```

r? `@oli-obk`

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