Rollup merge of #118135 - ouz-a:fix_stable_span, r=celinval

Remove quotation from filename in stable_mir

Previously we had quotation marks in filenames which is obviously wrong this fixes that.

r? ```@celinval```

compiler/stable_mir/src/lib.rs

@@ -110,7 +110,7 @@ pub enum ItemKind {
     Const,
 }
 
-pub type Filename = Opaque;
+pub type Filename = String;
 
 /// Holds information about an item in the crate.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]