Add a test for $x:T is vec<_>

Summary:
I was working on adding predicate-base refinement support for generic classes and discovered that it had issues with this case (due to supportdyn) which I guess wasn't already covered in the test cases.

This diff adds such a test.

Reviewed By: mheiber

Differential Revision: D76777602

fbshipit-source-id: be30ffa1305e7553d23244949fb08eb119b1b267

Author: viratyosin

hphp/hack/test/typecheck/refinements/generic_is_vec.php

@@ -0,0 +1,9 @@
+<?hh
+
+function takes_vec(vec<mixed> $x): void {}
+
+function foo<T>(T $x): void {
+  if ($x is vec<_>) {
+    takes_vec($x);
+  }
+}

hphp/hack/test/typecheck/refinements/generic_is_vec.php.exp

@@ -0,0 +1 @@
+No errors