[libc++] Refactor the Windows and MinGW implementation of the locale base API

This patch reimplements the locale base support for Windows flavors
in a way that is more modules-friendly and without defining non-internal
names.

Since this changes the name of some types and entry points in the
built library, this is effectively an ABI break on Windows (which
is acceptable since we don't promise ABI stability on that platform).

Author: ldionne

libcxx/docs/ReleaseNotes/20.rst

@@ -141,6 +141,9 @@ ABI Affecting Changes
   - When using the MSVC ABI, this change results in some classes having a completely different memory layout, so this is
     a genuine ABI break. However, the library does not currently guarantee ABI stability on MSVC platforms.
 
+- The localization support base API has been reimplemented, leading to different functions being exported from the
+  libc++ built library on Windows and Windows-like platforms.
+
 Build System Changes
 --------------------
 

libcxx/include/CMakeLists.txt

@@ -501,11 +501,11 @@ set(files
   __locale_dir/locale_base_api/ibm.h
   __locale_dir/locale_base_api/musl.h
   __locale_dir/locale_base_api/openbsd.h
-  __locale_dir/locale_base_api/win32.h
   __locale_dir/locale_guard.h
   __locale_dir/support/apple.h
   __locale_dir/support/bsd_like.h
   __locale_dir/support/freebsd.h
+  __locale_dir/support/windows.h
   __math/abs.h
   __math/copysign.h
   __math/error_functions.h

libcxx/include/__locale_dir/locale_base_api.h

@@ -98,15 +98,15 @@
 #  include <__locale_dir/support/apple.h>
 #elif defined(__FreeBSD__)
 #  include <__locale_dir/support/freebsd.h>
+#elif defined(_LIBCPP_MSVCRT_LIKE)
+#  include <__locale_dir/support/windows.h>
 #else
 
 // TODO: This is a temporary definition to bridge between the old way we defined the locale base API
 //       (by providing global non-reserved names) and the new API. As we move individual platforms
 //       towards the new way of defining the locale base API, this should disappear since each platform
 //       will define those directly.
-#  if defined(_LIBCPP_MSVCRT_LIKE)
-#    include <__locale_dir/locale_base_api/win32.h>
-#  elif defined(_AIX) || defined(__MVS__)
+#  if defined(_AIX) || defined(__MVS__)
 #    include <__locale_dir/locale_base_api/ibm.h>
 #  elif defined(__ANDROID__)
 #    include <__locale_dir/locale_base_api/android.h>