errors: span_suggestion takes impl ToString

Change `span_suggestion` (and variants) to take `impl ToString` rather
than `String` for the suggested code, as this simplifies the
requirements on the diagnostic derive.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -235,7 +235,7 @@ fn validate_default_attribute(
             .span_suggestion_hidden(
                 attr.span,
                 "try using `#[default]`",
-                "#[default]".into(),
+                "#[default]",
                 Applicability::MaybeIncorrect,
             )
             .emit();