Use line numbers relative to the function in mir-opt tests As shown in #99770, the line numbers can be a big source of needless and confusing diffs. This PR adds a new flag `-Zmir-pretty-relative-line-numbers` to make them relative to the function declaration, which avoids most needless diffs from attribute changes. `@JakobDegen` told me that there has been a zulip conversation about disabling line numbers with mixed opinions, so I'd like to get some feedback here, for this hopefully better solution. r? rust-lang/wg-mir-opt