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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@179173 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,chubbymaggie/asap,dslab-epfl/asap,dslab-epfl/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,apple/swift-llvm,chubbymaggie/asap,llvm-mirror/llvm,dslab-epfl/asap,apple/swift-llvm,apple/swift-llvm,chubbymaggie/asap,apple/swift-llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,chubbymaggie/asap,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm