c++: Add testcase for DR 2392

The testcase from DR 2392 passes, so I assume we don't need to do
anything further for the DR.

2022-11-15  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/DRs/dr2392.C: Add testcase for DR 2392.

gcc/testsuite/g++.dg/DRs/dr2392.C

@@ -0,0 +1,12 @@
+// DR 2392
+// { dg-do compile { target c++11 } }
+
+template <class T = void>
+constexpr int
+foo ()
+{
+  T t;
+  return 1;
+}
+
+using V = decltype (new int[foo ()]);