Add an -emit-sil-protocol-witness-tables flag.

So we can break stuff while it's turned on.

Swift SVN r10922

include/swift/Basic/LangOptions.h

@@ -39,6 +39,9 @@ namespace swift {
     
     /// \brief Enable codegen of lazy global initializers in SILGen.
     bool EmitLazyGlobalInitializers = false;
+    
+    /// \brief Enable generation of SIL protocol witness tables.
+    bool EmitSILProtocolWitnessTables = false;
   };
 }