[flang] Fix build error (unused data member warning)

flang/include/flang/Parser/provenance.h

public:
   bool Precedes(CharBlock, CharBlock) const;
 
   // Once a CookedSource is complete, add it to index_ and assign its number_
   void Register(CookedSource &);
 
 private:
   AllSources &allSources_;
   std::list<CookedSource> cooked_; // owns all CookedSource instances
-  int counter_{0};
   std::map<CharBlock, const CookedSource &, CharBlockPointerComparator> index_;
 };
 
 // For use as a Comparator for maps, sets, sorting, &c.
 class CharBlockComparator {
 public:
   explicit CharBlockComparator(const AllCookedSources &all) : all_{all} {}
   bool operator()(CharBlock x, CharBlock y) const {