Fixes#47094
Besides fixing that bug, this change has a user-visible effect on the spans in the "incompatible repr hints" warning and another error: they now point at `foo` and/or `bar` in `repr(foo, bar)` instead of the whole attribute. This is sometimes more precise (e.g., `#[repr(C, packed)]` on an enum points at the `packed`) but sometimes not. I moved a compile-fail test to a ui test to illustrate how it now looks in the common case of only one attribute.
