Improve tests for std::add_pointer

The type trait depends on is_void so the tests should check those cases.

	* testsuite/20_util/add_pointer/value.cc: Check void types.

From-SVN: r277672

libstdc++-v3/ChangeLog

 2019-10-31  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/20_util/add_pointer/value.cc: Check void types.
+
 	* include/bits/range_access.h (__sizable): Rename to __sentinel_size.
 	Remove workaround for PR c++/92268 and remove redundant requirements
 	that are already checked by sized_sentinel_for.

libstdc++-v3/testsuite/20_util/add_pointer/value.cc

@@ -31,8 +31,14 @@ void test01()
   static_assert(is_same<add_pointer<const int>::type, const int*>::value, "");
   static_assert(is_same<add_pointer<int&>::type, int*>::value, "");
   static_assert(is_same<add_pointer<ClassType*>::type,
-		ClassType**>::value, "");
+			ClassType**>::value, "");
   static_assert(is_same<add_pointer<ClassType>::type, ClassType*>::value, "");
+  static_assert(is_same<add_pointer<void>::type, void*>::value, "");
+  static_assert(is_same<add_pointer<const void>::type, const void*>::value, "");
+  static_assert(is_same<add_pointer<volatile void>::type,
+			volatile void*>::value, "");
+  static_assert(is_same<add_pointer<const volatile void>::type,
+			const volatile void*>::value, "");
 }
 
 void test02()