[test] Use lit's shell test runner on Windows

Summary:
I did a local comparison between using bash and using lit's runner, and
more of the suite passes with lit than passes with bash.  Most of the
bash failures have to do with /dev/null, which is nonsensical on
Windows, but the lit runner handles it.

The lit shell runner is also much faster than bash, so I would expect
most Windows devs would want it by default.

The behavior can be overridden on any OS by setting
LIT_USE_INTERNAL_SHELL to 0 or 1 in the environment.

Reviewers: chapuni, ddunbar

CC: llvm-commits, timurrrr

Differential Revision: http://llvm-reviews.chandlerc.com/D559