Rollup merge of #138685 - nnethercote:use-Option-Ident-for-lowered-param-names, r=compiler-errors

Use `Option<Ident>` for lowered param names.

Parameter patterns are lowered to an `Ident` by `lower_fn_params_to_names`, which is used when lowering bare function types, trait methods, and foreign functions. Currently, there are two exceptional cases where the lowered param can become an empty `Ident`.

- If the incoming pattern is an empty `Ident`. This occurs if the parameter is anonymous, e.g. in a bare function type.

- If the incoming pattern is neither an ident nor an underscore. Any such parameter will have triggered a compile error (hence the `span_delayed_bug`), but lowering still occurs.

This commit replaces these empty `Ident` results with `None`, which eliminates a number of `kw::Empty` uses, and makes it impossible to fail to check for these exceptional cases.

Note: the `FIXME` comment in `is_unwrap_or_empty_symbol` is removed. It actually should have been removed in #138482, the precursor to this PR. That PR changed the lowering of wild patterns to `_` symbols instead of empty symbols, which made the mentioned underscore check load-bearing.

r? ``@compiler-errors``

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -1992,13 +1992,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 .iter()
                 .enumerate()
                 .map(|(i, ident)| {
-                    if ident.name.is_empty()
-                        || ident.name == kw::Underscore
-                        || ident.name == kw::SelfLower
+                    if let Some(ident) = ident
+                        && !matches!(ident, Ident { name: kw::Underscore | kw::SelfLower, .. })
                     {
-                        format!("arg{i}")
-                    } else {
                         format!("{ident}")
+                    } else {
+                        format!("arg{i}")
                     }
                 })
                 .collect();