Add test case for revision 207575 to verify that on Windows, clang doesn't
crash with an assertion failure when 'nul' is passed in input.

Modified clang/test/lit.py to add feature 'system-windows' if
`platform.system()` returns 'Windows'.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207576 91177308-0d34-0410-b5e6-96231b3b80d8