Make string_to_string emit a suggestion when cloned can be used

tests/ui/string_to_string_in_map.fixed

@@ -0,0 +1,17 @@
+#![deny(clippy::string_to_string)]
+#![allow(clippy::unnecessary_literal_unwrap, clippy::useless_vec, clippy::iter_cloned_collect)]
+
+fn main() {
+    let variable1 = String::new();
+    let v = &variable1;
+    let variable2 = Some(v);
+    let _ = variable2.cloned();
+    //~^ string_to_string
+    let _ = variable2.cloned();
+    //~^ string_to_string
+
+    let _ = vec![String::new()].iter().cloned().collect::<Vec<_>>();
+    //~^ string_to_string
+    let _ = vec![String::new()].iter().cloned().collect::<Vec<_>>();
+    //~^ string_to_string
+}