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().
Reference: https://bugs.python.org/issue11395

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

llvm-svn: 256892