[libcxx] Finish localization support for LLVM libc

This patch enables building libcxx with LIBCXX_ENABLE_LOCALIZATION=ON
when building against LLVM libc.

This finishes up llvm#97508.

Author: boomanaiden154

libcxx/include/__locale_dir/messages.h

@@ -22,7 +22,7 @@
 
 #  if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 // Most unix variants have catopen.  These are the specific ones that don't.
-#    if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION) && !defined(__EMSCRIPTEN__)
+#    if !defined(__BIONIC__) && !defined(_NEWLIB_VERSION) && !defined(__EMSCRIPTEN__) && !defined(__LLVM_LIBC__)
 #      define _LIBCPP_HAS_CATOPEN 1
 #      include <nl_types.h>
 #    else