SILGen: Consistently use the buildMainActorExecutorRef built-in.
SILGen and IRGen would disagree on the return type of the
`swift_task_getMainExecutor()` runtime function if
`SILGenModule::getGetMainExecutor()` had ever been called. To address this,
consistently use the `buildMainActorExecutorRef` built-in and get rid of
`SILGenModule::getGetMainExecutor()`.
Resolves rdar://116472583
