cg: add explicit test for const param promotion

src/test/ui/const-generics/promotion.rs

@@ -0,0 +1,11 @@
+// run-pass
+// tests that promoting expressions containing const parameters is allowed.
+#![feature(min_const_generics)]
+
+fn promotion_test<const N: usize>() -> &'static usize {
+    &(3 + N)
+}
+
+fn main() {
+    assert_eq!(promotion_test::<13>(), &16);
+}