Summary:
The file itself will still get removed when `TemporaryFile` is destroyed, but closing it before then allows it to be read when on Windows, because `TemporaryFile` opens the temporary file with `O_EXCL`, so Windows denies read access of the file to everything else.
This is needed to fix one of the AsyncFileWriter tests on Windows.

Reviewed By: simpkins

Differential Revision: D5307719

fbshipit-source-id: bd65962cc3d34f382cc7b0b55dbf2659ed5ebfce