Enable serialization of witness tables by default

Now that we remove the [serialized] flag from functions after their early serialization, we can run another round of optimizations on them. Due to this change, it should be OK to serialize witness tables now because marking the witness methods [serialized] does not affect how well they can be optimized.

Author: swiftix

include/swift/AST/SILOptions.h

@@ -154,7 +154,7 @@ class SILOptions {
   ///
   /// It is supposed to be used only for compiling overlays.
   /// User code should never be compiled with this flag set.
-  bool SILSerializeWitnessTables = false;
+  bool SILSerializeWitnessTables = true;
 
   /// If set, SIL vtables will be serialized.
   ///