Add Builtin.hopToActor

SILGen this builtin to a mandatory hop_to_executor with an actor type
operand.

e.g.

    Task.detached {
      Builtin.hopToActor(MainActor.shared)
      await suspend()
    }

Required to fix a bug in _runAsyncMain.

lib/SIL/IR/SILPrinter.cpp

@@ -2154,6 +2154,8 @@ public:
   }
 
   void visitHopToExecutorInst(HopToExecutorInst *HTEI) {
+    if (HTEI->isMandatory())
+      *this << "[mandatory] ";
     *this << getIDAndType(HTEI->getTargetExecutor());
   }