Auto merge of #139131 - m-ou-se:format-args-struct-expr, r=Mark-Simulacrum

Simplify expansion for format_args!().

Instead of calling `Placeholder::new()`, we can just use a struct expression directly.

Before:

```rust
        Placeholder::new(…, …, …, …)
```

After:

```rust
        Placeholder {
                position: …,
                flags: …,
                width: …,
                precision: …,
        }
```

(I originally avoided the struct expression, because `Placeholder` had a lot of fields. But now that rust-lang/rust#136974 is merged, it only has four fields left.)

This will make the `fmt` argument to `fmt::Arguments::new_v1_formatted()` a candidate for const promotion, which is important if we ever hope to tackle rust-lang/rust#92698 (It doesn't change anything yet though, because the `args` argument to `fmt::Arguments::new_v1_formatted()` is not const-promotable.)

Author: bors