Fix various GCC mis-configurations for newer versions.

This patch goes through and enables C++11 and C++14 features for newer GCC's.
The main changes are:

1. Turn on variable templates. (Uses __cpp_variable_templates)
2. Assert atomic<Tp> is trivially copyable (Uses _GNUC_VER >= 501).
3. Turn on trailing return support for GCC. (Uses _GNUC_VER >= 404)
4. XFAIL void_t test for GCC 5.1 and 5.2. Fixed in GCC 6.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@255585 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/meta/meta.unary.prop.query/void_t.pass.cpp

@@ -11,11 +11,11 @@
 
 // void_t
 
-#include <type_traits>
+// UNSUPPORTED: c++98, c++03, c++11, c++14
 
-#if _LIBCPP_STD_VER <= 14
-int main () {}
-#else
+// XFAIL: gcc-5.1 gcc-5.2
+
+#include <type_traits>
 
 template <class T>
 void test1()
@@ -66,4 +66,3 @@ int main()
 
     static_assert( std::is_same<void, std::void_t<int, double const &, Class, volatile int[], void>>::value, "");
 }
-#endif