use more efficient code to generate repeated string

see https://rust.godbolt.org/z/z9vrFP for comparison

clippy_lints/src/strings.rs

@@ -154,7 +154,7 @@ fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, e: &'tcx Expr) {
                     if let LitKind::Str(ref lit_content, style) = lit.node {
                         let callsite = snippet(cx, args[0].span.source_callsite(), r#""foo""#);
                         let expanded = if let StrStyle::Raw(n) = style {
-                            let term = (0..n).map(|_| '#').collect::<String>();
+                            let term = "#".repeat(n as usize);
                             format!("r{0}\"{1}\"{0}", term, lit_content.as_str())
                         } else {
                             format!("\"{}\"", lit_content.as_str())