Use musttail when merging swifttail calls.

Fixes rdar://75899459.

Author: varungandhi-apple

lib/LLVMPasses/LLVMMergeFunctions.cpp

@@ -1225,7 +1225,11 @@ void SwiftMergeFunctions::writeThunk(Function *ToFunc, Function *Thunk,
   }
 
   CallInst *CI = Builder.CreateCall(ToFunc, Args);
-  CI->setTailCall();
+  bool isSwiftTailCall =
+   ToFunc->getCallingConv() == CallingConv::SwiftTail &&
+   Thunk->getCallingConv() == CallingConv::SwiftTail;
+  CI->setTailCallKind(
+    isSwiftTailCall ? llvm::CallInst::TCK_MustTail : llvm::CallInst::TCK_Tail);
   CI->setCallingConv(ToFunc->getCallingConv());
   CI->setAttributes(ToFunc->getAttributes());
   if (Thunk->getReturnType()->isVoidTy()) {