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_borrowck/src/diagnostics/mutability_errors.rs

@@ -237,7 +237,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     err.span_suggestion_verbose(
                         span,
                         "consider changing this to be mutable",
-                        " mut ".into(),
+                        " mut ",
                         Applicability::MaybeIncorrect,
                     );
                 }