Revision 1859732

io: Implement the functions that check for node existence using the native
Win32 APIs on Windows:

  svn_io_check_path()
  svn_io_check_resolved_path()
  svn_io_check_special_path()

This changeset aims for two distinct things:

1) First of all, starting from r1833621, these functions were patched to stop
   checking symlinks on Windows.  The root cause for this change has been an
   incorrect implementation of stat in APR that didn't properly distinguish
   between various types of reparse points, some of which are not symlinks.

   Providing the custom implementation allows us to remove the hack and
   properly handle such reparse points irrespectively of the APR version
   in use.

   Additional details on the core of the issue can be found here:
   - https://github.com/golang/go/issues/23684
   - https://github.com/dotnet/corefx/issues/24250

2) These APIs are used in various performance-critical code paths such as
   in the core part of `svn st`.

   On Win32 the necessary answers for these particular functions can be
   obtained with a single call to GetFileAttributes(), which is much faster
   than using the generic stat implementations from APR 1.6.x and 1.7.x
   (I believe that it would be even slower in the latter case).

   A couple of quick tests show about 20%-25% improvement in the speed of
   `svn st` for a large working copy.  The improvement may be more significant
   with indexers or virus scanners, as just asking for file attributes may
   completely avoid opening a file (by retrieving the result through the
   fast I/O QueryOpen).


* subversion/libsvn_subr/io.c
  (FILE_ATTRIBUTE_TAG_INFO, FileAttributeTagInfo): Add these definitions
   for old versions of Windows SDK.
  (typedef GetFileInformationByHandleEx_t,
   get_file_information_by_handle_ex_proc): New.
  (win_init_dynamic_imports): Import `GetFileInformationByHandleEx()`.
  (win32_get_file_information_by_handle): New helper function.
  (io_win_check_path): New helper with the Win32 implementation required
   for the "check path" functions.
  (svn_io_check_path,
   svn_io_check_resolved_path,
   svn_io_check_special_path): Invoke the new helper.
  (io_check_path): Undo the workaround from r1833621 that stopped passing
   APR_FINFO_LINK when performing a stat.