Revert "Revert "Merge to upstream r304942.""

This reverts commit 38a0d5af7e.

Test: make checkbuild
Test: ./run_tests.py
Test: ./run_tests.py --bitness 64

test/std/strings/basic.string/traits_mismatch.fail.cpp

@@ -0,0 +1,18 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+//   The strings's value type must be the same as the traits's char_type
+
+#include <string>
+
+int main()
+{
+    std::basic_string<char, std::char_traits<wchar_t>> s;
+}