Implement LWG#2761: 'basic_string should require that charT match traits::char_type'. Tests for string_view, too

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@297872 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -7,6 +7,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+// <string_view>
+//   The string_views's value type must be the same as the traits's char_type
+
 #include <string_view>
 
-int main () {}
+int main()
+{
+    std::basic_string_view<char, std::char_traits<wchar_t>> s;
+}