rustc_middle: Fix opt_item_ident for non-local def ids

tests/ui/error-codes/E0277.stderr

@@ -5,7 +5,8 @@ LL | fn f(p: Path) { }
    |      ^ doesn't have a size known at compile-time
    |
    = help: within `Path`, the trait `Sized` is not implemented for `[u8]`
-   = note: required because it appears within the type `Path`
+note: required because it appears within the type `Path`
+  --> $SRC_DIR/std/src/path.rs:LL:COL
    = help: unsized fn params are gated as an unstable feature
 help: function arguments must have a statically known size, borrowed types always have a known size
    |