Make string_to_string emit a suggestion when cloned can be used

tests/ui/string_to_string.rs

@@ -1,8 +1,22 @@
 #![warn(clippy::string_to_string)]
-#![allow(clippy::redundant_clone)]
+#![allow(clippy::redundant_clone, clippy::unnecessary_literal_unwrap)]
 
 fn main() {
     let mut message = String::from("Hello");
     let mut v = message.to_string();
     //~^ string_to_string
+
+    let variable1 = String::new();
+    let v = &variable1;
+    let variable2 = Some(v);
+    let _ = variable2.map(|x| {
+        println!();
+        x.to_string()
+    });
+    //~^^ string_to_string
+
+    // Should not lint.
+    let x = Some(String::new());
+    let _ = x.unwrap_or_else(|| v.to_string());
+    //~^ string_to_string
 }