Auto merge of #117094 - Nadrieril:warn-lint-on-arm, r=cjgillot

Warn users who set `non_exhaustive_omitted_patterns` lint level on a match arm

Before https://github.com/rust-lang/rust/pull/116734, the recommended usage of the [`non_exhaustive_omitted_patterns` lint](https://github.com/rust-lang/rust/issues/89554) was:
```rust
match Bar::A {
    Bar::A => {},
    #[warn(non_exhaustive_omitted_patterns)]
    _ => {},
}
```
After https://github.com/rust-lang/rust/pull/116734 this no longer makes sense, and recommended usage is now:
```rust
#[warn(non_exhaustive_omitted_patterns)]
match Bar::A {
    Bar::A => {},
    _ => {},
}
```

As you can guess, this silently breaks all uses of the lint that used the previous form. This is a problem in particular because `syn` recommends usage of this lint to its users in the old way. This PR emits a warning when the previous form is used so users can update.

r? `@cjgillot`

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