Add a cl.exe compatible driver mode

The mode doesn't actually do anything yet, but this provides a
way to get into it.

llvm-svn: 186720
diff --git a/clang/test/Driver/cl.c b/clang/test/Driver/cl.c
new file mode 100644
index 0000000..c527313
--- /dev/null
+++ b/clang/test/Driver/cl.c
@@ -0,0 +1,3 @@
+// RUN: %clang_cl -fsyntax-only -c %s
+
+void f();