[libc++] Reenable codecvt in the dylib. (#73679)

The header is used in the dylib, this is not an issue at the moment
since the dylib is built using C++23 but it would be when building
with C++26.

Post release comments in #72496 seem to indicate this removal is an
issue for Fuchsia.

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>

libcxx/test/libcxx/input.output/file.streams/fstreams/fstream.cons/wchar_pointer.pass.cpp

@@ -17,6 +17,9 @@
 // REQUIRES: windows
 // UNSUPPORTED: no-wide-characters
 
+// TODO: This should not be necessary
+// ADDITIONAL_COMPILE_FLAGS:-D_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT
+
 #include <fstream>
 #include <cassert>
 #include "test_macros.h"

libcxx/test/libcxx/input.output/file.streams/fstreams/fstream.members/open_wchar_pointer.pass.cpp

@@ -17,6 +17,9 @@
 // REQUIRES: windows
 // UNSUPPORTED: no-wide-characters
 
+// TODO: This should not be necessary
+// ADDITIONAL_COMPILE_FLAGS:-D_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT
+
 #include <fstream>
 #include <cassert>
 #include "test_macros.h"

libcxx/test/libcxx/input.output/file.streams/fstreams/ofstream.cons/wchar_pointer.pass.cpp

@@ -17,6 +17,9 @@
 // REQUIRES: windows
 // UNSUPPORTED: no-wide-characters
 
+// TODO: This should not be necessary
+// ADDITIONAL_COMPILE_FLAGS:-D_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT
+
 #include <fstream>
 #include <cassert>
 #include "test_macros.h"

libcxx/test/libcxx/input.output/file.streams/fstreams/ofstream.members/open_wchar_pointer.pass.cpp

@@ -17,6 +17,9 @@
 // REQUIRES: windows
 // UNSUPPORTED: no-wide-characters
 
+// TODO: This should not be necessary
+// ADDITIONAL_COMPILE_FLAGS:-D_LIBCPP_ENABLE_CXX26_REMOVED_CODECVT
+
 #include <fstream>
 #include <cassert>
 #include "test_macros.h"