Add a cl.exe compatible driver mode

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186720 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Driver.cpp b/lib/Driver/Driver.cpp
index 0c3f6ca..98b87be 100644
--- a/lib/Driver/Driver.cpp
+++ b/lib/Driver/Driver.cpp
@@ -96,6 +96,7 @@
         .Case("gcc", GCCMode)
         .Case("g++", GXXMode)
         .Case("cpp", CPPMode)
+        .Case("cl",  CLMode)
         .Default(~0U);
 
     if (M != ~0U)