[IRGen] Balance out a retain count for createAsyncTask's context.

The `createAsyncTask` entry point expects the parent task to be
"guaranteed" and the function to be "owned". However, that's not easy
to model in the operand ownership map, so we consider all operands to
be "guaranteed" and balance out the reference count when lowering the builtin.

This is an egregious hack that will create a little extra reference
count traffic.

Author: DougGregor

lib/IRGen/GenBuiltin.cpp

@@ -230,6 +230,12 @@ void irgen::emitBuiltinCall(IRGenFunction &IGF, const BuiltinInfo &Builtin,
     auto parentTask = args.claimNext();
     auto taskFunction = args.claimNext();
     auto taskContext = args.claimNext();
+
+    // FIXME: SIL treats the function/context parameter as "guaranteed", but
+    // the runtime entry point assumes it is owned. Introduce an extra retain
+    // of the context to balance things out.
+    IGF.emitNativeStrongRetain(taskContext, IGF.getDefaultAtomicity());
+
     auto newTaskAndContext = emitTaskCreate(
         IGF, flags, parentTask, taskFunction, taskContext);
 

test/IRGen/async/builtins.sil

@@ -33,11 +33,12 @@ bb0(%0 : @guaranteed $Builtin.NativeObject):
 sil hidden [ossa] @launch_task : $@convention(method) @async (Int, Optional<Builtin.NativeObject>, @guaranteed @async @callee_guaranteed () -> (@error Error)) -> () {
 bb0(%0 : $Int, %1: @unowned $Optional<Builtin.NativeObject>, %2: @guaranteed $@async @callee_guaranteed () -> (@error Error)):
   %3 = begin_borrow %1 : $Optional<Builtin.NativeObject>
+  // CHECK: call %swift.refcounted* @swift_retain(%swift.refcounted* returned [[FN_CONTEXT:%.*]])
   // CHECK: [[NEW_TASK_AND_CONTEXT:%.*]] = call swiftcc %swift.async_task_and_context @swift_task_create_f
   // CHECK-NEXT: [[NEW_CONTEXT_RAW:%.*]] = extractvalue %swift.async_task_and_context [[NEW_TASK_AND_CONTEXT]], 1
   // CHECK-NEXT: [[NEW_CONTEXT:%.*]] = bitcast %swift.context* [[NEW_CONTEXT_RAW]] to
   // CHECK-NEXT: [[CONTEXT_INFO_LOC:%.*]] = getelementptr inbounds <{{.*}}>* [[NEW_CONTEXT]]
-  // CHECK-NEXT: store %swift.refcounted* %{{.*}}, %swift.refcounted** [[CONTEXT_INFO_LOC]]
+  // CHECK-NEXT: store %swift.refcounted* [[FN_CONTEXT]], %swift.refcounted** [[CONTEXT_INFO_LOC]]
   %20 = builtin "createAsyncTask"(%0 : $Int, %3 : $Optional<Builtin.NativeObject>, %2 : $@async @callee_guaranteed () -> (@error Error)) : $(Builtin.NativeObject, Builtin.RawPointer)
   end_borrow %3 : $Optional<Builtin.NativeObject>
   destroy_value %20 : $(Builtin.NativeObject, Builtin.RawPointer)