Add test for promotability in let

The old const-checker conservatively reset qualifs when
`IsNotPromotable` was in the return place. Unfortunately, named
variables have `IsNotPromotable`, so this could cause promotion to fail.
This should work now.

src/test/ui/consts/const_let_promote.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+use std::cell::Cell;
+
+const X: Option<Cell<i32>> = None;
+
+const Y: Option<Cell<i32>> = {
+    let x = None;
+    x
+};
+
+// Ensure that binding the final value of a `const` to a variable does not affect promotion.
+#[allow(unused)]
+fn main() {
+    let x: &'static _ = &X;
+    let y: &'static _ = &Y;
+}