Handle missing members in protocols as well.

This means both not crashing when we deserialize the protocol but
also emitting correct offsets for dynamic dispatch through the
protocol's witness table.

Also fix a bug with vtable and witness table slots for
materializeForSet accessors for properties that can't be
imported. Because materializeForSet doesn't have the type of the
property in its signature, it was taking a different failure path from
everything else, and that failure path didn't properly set the name or
flags for the missing member.

Finishes rdar://problem/31878396

include/swift/SIL/SILWitnessVisitor.h

@@ -143,6 +143,10 @@ public:
     asDerived().addMethod(func);
   }
 
+  void visitMissingMemberDecl(MissingMemberDecl *placeholder) {
+    asDerived().addPlaceholder(placeholder);
+  }
+
   void visitAssociatedTypeDecl(AssociatedTypeDecl *td) {
     // We already visited these in the first pass.
   }