[libcxx] Remove use of default function template parameters in type traits. Fixes DR20484

Summary: This patch moves the SFINAE for __is_destructor_welformed out of the function template parameters. type_traits must compile in c++03 mode since it is included in c++03 headers. 

Test Plan: No tests have been added.

Reviewers: danalbert, mclow.lists

Reviewed By: danalbert

Subscribers: K-ballo, cfe-commits

Differential Revision: http://reviews.llvm.org/D4735

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/branches/release_35@214427 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/type_traits b/include/type_traits
index e9f6c01..371a15e 100644
--- a/include/type_traits
+++ b/include/type_traits
@@ -1544,16 +1544,21 @@
 //	Otherwise, return "std::declval<_Up&>().~_Up()" is well-formed
 //    where _Up is remove_all_extents<_Tp>::type
+template <class>
+struct __is_destructible_apply { typedef int type; };
 template <typename _Tp>
 struct __is_destructor_wellformed {
-	template <typename _Tp1, typename _Tp2 = decltype(_VSTD::declval<_Tp1&>().~_Tp1())>
-	static char  __test (int);
+	template <typename _Tp1>
+	static char  __test (
+        typename __is_destructible_apply<decltype(_VSTD::declval<_Tp1&>().~_Tp1())>::type
+    );
 	template <typename _Tp1>
 	static __two __test (...);
 	static const bool value = sizeof(__test<_Tp>(12)) == sizeof(char);
-	};
 template <class _Tp, bool>
 struct __destructible_imp;