Optimize KWSys SystemTools::FileExists on Windows
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.
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