lint: only consider actual calls as unconditional recursion.

Specifically, just mentioning the function name as a value is fine, as
long as it isn't called, e.g. `fn main() { let _ = main; }`.

Closes #21705.

src/test/compile-fail/lint-unconditional-recursion.rs

@@ -63,4 +63,8 @@ impl Baz {
     }
 }
 
+fn all_fine() {
+    let _f = all_fine;
+}
+
 fn main() {}