[libc] Do not include langinfo.h when using the LLVM C library

Summary:
The `langinfo.h` header is a POSIX extension, so ideally we would be
able to build the C++ library without it. Currently the LLVM C library
doesn't support / provide it. This allows us to build the C++ library
with locales enabled. We can either disable it here, or just provide
stubs that do nothing as in #106620.

Author: jhuber6

libcxx/src/locale.cpp

@@ -34,7 +34,8 @@
 #  define _CTYPE_DISABLE_MACROS
 #endif
 
-#if !defined(_LIBCPP_MSVCRT) && !defined(__MINGW32__) && !defined(__BIONIC__) && !defined(__NuttX__)
+#if !defined(_LIBCPP_MSVCRT) && !defined(__MINGW32__) && !defined(__BIONIC__) && !defined(__NuttX__) ||                \
+    defined(__LLVM_LIBC__)
 #  include <langinfo.h>
 #endif