[ORC] Re-add the Windows check that was dropped in r328687.

This check prevents the ORC execution tests from running on Windows (which is
not supported yet).

This should fix the windows bots.

llvm-svn: 328706
diff --git a/llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.h b/llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.h
index 108e275..53fede1 100644
--- a/llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.h
+++ b/llvm/unittests/ExecutionEngine/Orc/OrcTestCommon.h
@@ -61,6 +61,10 @@
       // If we found a TargetMachine, check that it's one that Orc supports.
       const Triple& TT = TM->getTargetTriple();
 
+      // Bail out for windows platforms. We do not support these yet.
+      if (TT.isOSWindows())
+        return;
+
       // Target can JIT?
       SupportsJIT = TM->getTarget().hasJIT();
       // Use ability to create callback manager to detect whether Orc