Don't disable stdin buffering on Windows

Disabling buffering exposes a bug in the MS VS 2015 CRT implementation of fgets, where you sometimes have to hit Enter twice, depending on if the input had an odd or even number of characters.

This was hidden until a few days ago by the Python initialization which was re-enabling buffering on the streams. A few days ago, Enrico make the Python initialization on-demand, which exposed this problem.

llvm-svn: 266384