[flang] Fix build bot problem

[klausler]

Summary

A recent change is eliciting a valid warning from the out-of-tree
flang build bot; fix by using a reference in a range-based for().

flang/lib/Lower/ConvertVariable.cpp

void Fortran::lower::defineCommonBlocks(
     Fortran::lower::AbstractConverter &converter,
     const Fortran::semantics::CommonBlockList &commonBlocks) {
   // Common blocks may depend on another common block address (if they contain
   // pointers with initial targets). To cover this case, create all common block
   // fir::Global before creating the initial values (if any).
   std::vector<std::tuple<fir::GlobalOp, Fortran::semantics::MutableSymbolVector,
                          mlir::Location>>
       delayedInitializations;
-  for (const auto [common, size] : commonBlocks)
+  for (const auto &[common, size] : commonBlocks)
     if (auto delayedInit = declareCommonBlock(converter, common, size))
       delayedInitializations.emplace_back(std::move(*delayedInit));
   for (auto &[global, cmnBlkMems, loc] : delayedInitializations)
     finalizeCommonBlockDefinition(loc, converter, global, cmnBlkMems);
 }
 
 /// The COMMON block is a global structure. `var` will be at some offset
 /// within the COMMON block. Adds the address of `var` (COMMON + offset) to