Use lit's internal shell runner on Windows for Clang tests

The behavior can be overridden by setting LIT_USE_INTERNAL_SHELL=0 in
the environment.

This fixes issues with /dev/null for me and brings the test suite time
down to 30s.  =D

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179283 91177308-0d34-0410-b5e6-96231b3b80d8