Use musttail in MergeFunctions for swiftasync->swiftasync tail calls.

Fixes rdar://76024125.

Author: varungandhi-apple

llvm/lib/Transforms/IPO/MergeFunctions.cpp

@@ -713,7 +713,11 @@ void MergeFunctions::writeThunk(Function *F, Function *G) {
 
   CallInst *CI = Builder.CreateCall(F, Args);
   ReturnInst *RI = nullptr;
-  CI->setTailCall();
+  bool isSwiftTailCall =
+   F->getCallingConv() == CallingConv::SwiftTail &&
+   G->getCallingConv() == CallingConv::SwiftTail;
+  CI->setTailCallKind(
+    isSwiftTailCall ? llvm::CallInst::TCK_MustTail : llvm::CallInst::TCK_Tail);
   CI->setCallingConv(F->getCallingConv());
   CI->setAttributes(F->getAttributes());
   if (H->getReturnType()->isVoidTy()) {