Don't run dwo tests for windows targets.

-gsplit-dwarf is not implemented by clang on Windows.  As such,
all the dwo tests are having the -gsplit-dwarf command line option
completely ignored, and the result is you get regular dwarf debug
information, and it's just running the exact same tests twice,
doubling the length of the test suite for no good reason.

llvm-svn: 257363