[FuzzMutate] Match the callee's and callsite's calling conventions. (llvm#139100)

The mutator can generate calls with undefined behavior because it is not
matching the calling conventions.

Author: mgcarrasco

llvm/lib/FuzzMutate/IRMutator.cpp

@@ -391,6 +391,7 @@ void InsertFunctionStrategy::mutate(BasicBlock &BB, RandomIRBuilder &IB) {
                                          BasicBlock::iterator InsertPt) {
     StringRef Name = isRetVoid ? nullptr : "C";
     CallInst *Call = CallInst::Create(FTy, F, Srcs, Name, InsertPt);
+    Call->setCallingConv(F->getCallingConv());
     // Don't return this call inst if it return void as it can't be sinked.
     return isRetVoid ? nullptr : Call;
   };