IPDL change: 'sync' parent-to-child messages are banhammered

Note I explicitly mean 'sync', this doesn't apply to 'rpc' messages.

If your protocol has a message declaration like

   protocol Foo {
   //...
   child:
     sync Msg();
   //...
   };

it will soon fail the IPDL type checker.

We are making this change to avoid having to statically analyze C++ 
protocol actors for potential deadlock across protocols (IPDL prevents 
intra-protocol deadlock), which was the original plan.  Pre-empting 
these at the IPDL level is better for a lot of reasons.

Cheers,
Chris