Add regression test for rust-lang#89432

Co-authored-by: Josh Stone <[email protected]>

Author: usbalbin

src/test/ui/consts/not_const_clusure_in_const.rs

@@ -0,0 +1,9 @@
+// run-pass
+
+const _FOO: fn() -> String = || "foo".into();
+
+pub fn bar() -> fn() -> String {
+    || "bar".into()
+}
+
+fn main(){}