Change a static_assert to check for is_trivial instead of is_pod, as is mandated by P0767.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323071 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string b/include/string
index f5d5489..b213cd4 100644
--- a/include/string
+++ b/include/string
@@ -651,7 +651,7 @@
     typedef typename __alloc_traits::pointer             pointer;
     typedef typename __alloc_traits::const_pointer       const_pointer;
 
-    static_assert(is_pod<value_type>::value, "Character type of basic_string must be a POD");
+    static_assert(is_trivial<value_type>::value, "Character type of basic_string must be trivial");
     static_assert((is_same<_CharT, typename traits_type::char_type>::value),
                   "traits_type::char_type must be the same type as CharT");
     static_assert((is_same<typename allocator_type::value_type, value_type>::value),