lit.cfg: better check for MSYS

When running the tests under Cygwin using non-Cygwin python,
the platform would be Windows and there would be bash on the path,
so this check for MSYS would not work correctly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187737 91177308-0d34-0410-b5e6-96231b3b80d8