Add a passing test for const unsafe_unreachable

src/test/ui/consts/const_unsafe_unreachable.rs

@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(const_fn)]
+#![feature(const_unreachable_unchecked)]
+
+const unsafe fn foo(x: bool) -> bool {
+    match x {
+        true => true,
+        false => std::hint::unreachable_unchecked(),
+    }
+}
+
+const BAR: bool = unsafe { foo(true) };
+
+fn main() {
+  assert_eq!(BAR, true);
+}