Rollup merge of #158859 - blyxyas:fix-ls-unstable-opt, r=bjorn3

Improve `-Zls` diagnostic message on `.rs` files

Refer the user to use `rustc -Zls` on `.rmeta` files instead of `.rs` files. And if they're using cargo, warn them to use rustc directly.

> [!IMPORTANT]
> First few comments in this pull request refer to changing the flag itself to work on source files. This was instead overruled to warn the user about executing on `.rmeta` files instead of `.rs` files.

<details><summary>Old description</summary>
<p>
This unstable option has been broken since forever (more than 2.5 years,
at least), but it's exactly what I need for debugging the new
incremental system, so rather than removing it I opted for fixing it.

The issue was that we were feeding the source file like, `src/lib.rs` instead of the `.rmeta` file. So literally every use of `-Zls=..` was returning  `invalid metadata version found: ..`

This PR also includes some testing.

Relevant for https://github.com/rust-lang/rust-project-goals/issues/641.

</p>
</details>