[windows] Always use the lit shell on Windows, even if bash is present

Summary:
This is consistent with LLVM and Clang. The lit shell isn't a complete
bash implementation, but its behavior is more easily reproducible.  This
fixes some ubsan test failures.

One ubsan test requires a shell currently, so I added "REQUIRES: shell",
and the other doesn't work on Windows because it prints a stack trace
and uses a linker that doesn't support DWARF. We can fix it eventually
through other means.

Reviewers: samsonov, pcc

Subscribers: yaron.keren, filcab, llvm-commits

Differential Revision: http://reviews.llvm.org/D11960

llvm-svn: 244837