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_expand/src/config.rs

@@ -511,7 +511,7 @@ pub fn parse_cfg<'a>(meta_item: &'a MetaItem, sess: &Session) -> Option<&'a Meta
             err.span_suggestion(
                 span,
                 "expected syntax is",
-                suggestion.into(),
+                suggestion,
                 Applicability::HasPlaceholders,
             );
         }