Replace elided variable in let_unit with () when used

Situation: `let_unit` lints when an expression binds a unit (`()`)
to a variable. In some cases this binding may be passed down to
another function. Currently, the lint removes the binding without
considering usage.

Change: All usages of the elided variable are now replaced with `()`.

fixes: #12594

tests/ui/let_unit.fixed

@@ -177,3 +177,21 @@ async fn issue10433() {
 }
 
 pub async fn issue11502(a: ()) {}
+
+pub fn issue12594() {
+    fn returns_unit() {}
+
+    fn returns_result<T>(res: T) -> Result<T, ()> {
+        Ok(res)
+    }
+
+    fn actual_test() {
+        // create first a unit value'd value
+        returns_unit();
+        returns_result(()).unwrap();
+        returns_result(()).unwrap();
+        // make sure we replace only the first variable
+        let res = 1;
+        returns_result(res).unwrap();
+    }
+}