[IRGen] Temporarily treat @async @convention(c) as not async.

Author: nate-chandler

lib/IRGen/IRGenSIL.cpp

@@ -1331,7 +1331,11 @@ std::unique_ptr<COrObjCEntryPointArgumentEmission>
 getCOrObjCEntryPointArgumentEmission(IRGenSILFunction &IGF,
                                      SILBasicBlock &entry,
                                      Explosion &allParamValues) {
-  if (IGF.CurSILFn->isAsync()) {
+  if (IGF.CurSILFn->isAsync() &&
+      !(/*FIXME: Remove this condition once Task.runDetached is
+                            available.  rdar://problem/70597390*/
+        IGF.CurSILFn->getLoweredFunctionType()->getRepresentation() ==
+        SILFunctionTypeRepresentation::CFunctionPointer)) {
     llvm_unreachable("unsupported");
   } else {
     return std::make_unique<SyncCOrObjCEntryPointArgumentEmission>(
@@ -3141,7 +3145,12 @@ static void emitReturnInst(IRGenSILFunction &IGF,
     auto &retTI = cast<LoadableTypeInfo>(IGF.getTypeInfo(resultTy));
     retTI.initialize(IGF, result, IGF.IndirectReturn, false);
     IGF.Builder.CreateRetVoid();
-  } else if (IGF.isAsync()) {
+  } else if (IGF.isAsync() &&
+             !(/*FIXME: Remove this condition once Task.runDetached is
+                  available.  rdar://problem/70597390*/
+               IGF.CurSILFn->getLoweredFunctionType()
+                   ->getRepresentation() ==
+               SILFunctionTypeRepresentation::CFunctionPointer)) {
     // If we're generating an async function, store the result into the buffer.
     assert(!IGF.IndirectReturn.isValid() &&
            "Formally direct results should stay direct results for async "