[CoroutineAccessors] Open-code a task_dealloc_thru

When building for back-deployment, emit calls to an open-coded
`_swift_task_dealloc_through` function rather than the runtime
`swift_task_dealloc_through` which doesn't exist on them.

Author: nate-chandler

lib/IRGen/CMakeLists.txt

@@ -17,6 +17,7 @@ add_swift_host_library(swiftIRGen STATIC
   GenClass.cpp
   GenConstant.cpp
   GenControl.cpp
+  GenCoro.cpp
   GenCoverage.cpp
   GenConcurrency.cpp
   GenDistributed.cpp