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_lint/src/array_into_iter.rs

@@ -129,7 +129,7 @@ impl<'tcx> LateLintPass<'tcx> for ArrayIntoIter {
                 diag.span_suggestion(
                     call.ident.span,
                     "use `.iter()` instead of `.into_iter()` to avoid ambiguity",
-                    "iter".into(),
+                    "iter",
                     Applicability::MachineApplicable,
                 );
                 if self.for_expr_span == expr.span {