LLVM 6.0.1p8 fails to compile on 6.4-stable/loongson

Hi,

As title says. LLVM 6.0.1p8 build fails due to missing header files.
The most relevant outputs:

#+BEGIN_QUOTE
In file included from /usr/ports/pobj/llvm-6.0.1/llvm-6.0.1.src/tools/lldb/source/Interpreter/CommandInterpreter.cpp:53:0:
/usr/ports/pobj/llvm-6.0.1/llvm-6.0.1.src/tools/lldb/include/lldb/Host/Editline.h:49:19: fatal error: codecvt: No such file or directory
#include <codecvt>
^
compilation terminated.
ninja: build stopped: subcommand failed.
#+END_QUOTE

The full build log is also attached. Please advice on how to fix or debug.

Thanks.

--
manphiz