Fixing PR25717: fatal IO error writing large outputs to console on Windows.
This patch is similar to the Python issue#11395. We need to cap the output
size to 32767 on Windows to work around the size limit of WriteConsole().
Writing a test for this bug turns out to be harder than I thought. I am
still working on it (see phabricator review D15705).
Differential Revision: http://reviews.llvm.org/D15553