Pull in r292830 from upstream libc++ trunk (by Eric Fiselier):

Fix GCC C++03 build by hiding default template argument in C++03

This allows the graphics/GraphicsMagic to compile with gcc 4.9 and
libc++.

PR:		216404

contrib/libc++/include/string

@@ -818,7 +818,10 @@ public:
     operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
 
     basic_string& operator=(const basic_string& __str);
+
+#ifndef _LIBCPP_CXX03_LANG
     template <class = void>
+#endif
     _LIBCPP_INLINE_VISIBILITY
     basic_string& operator=(__self_view __sv)  {return assign(__sv);}
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES