We optimize this method by using the GetFileAttributesExA native Windows
API to check for file existence when possible.  For real Windows builds
we always use it.  For Cygwin we use cygwin_conv_to_win32_path to get a
native Windows path if possible and otherwise fall back to 'access'.

Cygwin-to-Windows path conversion and cache by Wojciech Migda.
See issue #8826.