lit/Util.py: On Cygwin, 'PATHEXT' may exist but it should not be used.
lit/Util.py: On Cygwin, 'PATHEXT' may exist but it should not be used.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@125272 91177308-0d34-0410-b5e6-96231b3b80d8

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