[libc++] Reenable codecvt in the dylib.

The header is used in the dylib, this is not an issue at the moment
since the dylib is built using C++23.

Post release comments in #72496 seem to indicate this removal is an
issue for Fuchsia, this is a test so see whether it fixes the issue for
their builds.

Author: mordante

libcxx/include/codecvt

@@ -63,7 +63,7 @@ class codecvt_utf8_utf16
 #  pragma GCC system_header
 #endif
 
-#if _LIBCPP_STD_VER < 26 || defined(_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT)
+#if _LIBCPP_STD_VER < 26 || defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT)
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -555,7 +555,7 @@ _LIBCPP_SUPPRESS_DEPRECATED_POP
 
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // _LIBCPP_STD_VER < 26 || defined(_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT)
+#endif // _LIBCPP_STD_VER < 26 || defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT)
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <atomic>