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>

clippy_lints/src/functions/must_use.rs

@@ -108,7 +108,7 @@ fn check_needless_must_use(
                 diag.span_suggestion(
                     attr.span,
                     "remove the attribute",
-                    "".into(),
+                    "",
                     Applicability::MachineApplicable,
                 );
             },

clippy_lints/src/needless_pass_by_value.rs

@@ -241,7 +241,7 @@ fn check_fn(
                                                 |x| Cow::from(format!("change `{}` to", x)),
                                             )
                                             .as_ref(),
-                                        suggestion.into(),
+                                        suggestion,
                                         Applicability::Unspecified,
                                     );
                                 }
@@ -271,7 +271,7 @@ fn check_fn(
                                                 |x| Cow::from(format!("change `{}` to", x))
                                             )
                                             .as_ref(),
-                                        suggestion.into(),
+                                        suggestion,
                                         Applicability::Unspecified,
                                     );
                                 }