[flang] Make the symbol count used for collating symbols static

[PeteSteinfeld]

Summary

I changed the declaration of symbolCount_ in the type Symbols to be
static to avoid possible problems in the future when we might have
multiple objects of type Symbols. Thanks to Peter for pointing out the
need for this change.

[PeteSteinfeld]

Ping

flang/include/flang/Semantics/symbol.h

Symbol &Make(const Scope &owner, const SourceName &name, const Attrs &attrs,
     return symbol;
   }
 
 private:
   using blockType = std::array<Symbol, BLOCK_SIZE>;
   std::list<blockType *> blocks_;
   std::size_t nextIndex_{0};
   blockType *currBlock_{nullptr};
-  std::size_t symbolCount_ = 0;
+  static inline std::size_t symbolCount_ = 0;
 
   Symbol &Get() {
     if (nextIndex_ == 0) {
       blocks_.push_back(new blockType());
       currBlock_ = blocks_.back();
     }
     Symbol &result = (*currBlock_)[nextIndex_];
     if (++nextIndex_ >= BLOCK_SIZE) {