[DeadArgElim] Use structure bindings in foreach loops. NFC

[psamolysov]

Summary

Since LLVM supports C++17, new features from the standard such as structure binding can be used in the source code.

[nlopes]

This kind of patches don't need pre-commit review.

[psamolysov]

@nlopes Thank you for the information. I've opened the review to get whether this kind of c++17tion makes any sense at all.

llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp

for (Function::arg_iterator I = F.arg_begin(), E = F.arg_end(),
     // Move the name and users over to the new version.
     I->replaceAllUsesWith(&*I2);
     I2->takeName(&*I);
   }
 
   // Clone metadata from the old function, including debug info descriptor.
   SmallVector<std::pair<unsigned, MDNode *>, 1> MDs;
   F.getAllMetadata(MDs);
-  for (auto MD : MDs)
-    NF->addMetadata(MD.first, *MD.second);
+  for (auto [KindID, Node] : MDs)
+    NF->addMetadata(KindID, *Node);
 
   // Fix up any BlockAddresses that refer to the function.
   F.replaceAllUsesWith(ConstantExpr::getBitCast(NF, F.getType()));
   // Delete the bitcast that we just created, so that NF does not
   // appear to be address-taken.
   NF->removeDeadConstantUsers();
   // Finally, nuke the old function.
   F.eraseFromParent();
for (BasicBlock &BB : *NF)
         auto *NewRet = ReturnInst::Create(F->getContext(), RetVal, RI);
         NewRet->setDebugLoc(RI->getDebugLoc());
         BB.getInstList().erase(RI);
       }
 
   // Clone metadata from the old function, including debug info descriptor.
   SmallVector<std::pair<unsigned, MDNode *>, 1> MDs;
   F->getAllMetadata(MDs);
-  for (auto MD : MDs)
-    NF->addMetadata(MD.first, *MD.second);
+  for (auto [KindID, Node] : MDs)
+    NF->addMetadata(KindID, *Node);
 
   // If either the return value(s) or argument(s) are removed, then probably the
   // function does not follow standard calling conventions anymore. Hence, add
   // DW_CC_nocall to DISubroutineType to inform debugger that it may not be safe
   // to call this function or try to interpret the return value.
   if (NFTy != FTy && NF->getSubprogram()) {
     DISubprogram *SP = NF->getSubprogram();
     auto Temp = SP->getType()->cloneWithCC(llvm::dwarf::DW_CC_nocall);