[clang-tidy][NFC] Remove Tristate from CachedGlobList

[njames93]

Summary

The tristate is a little redundant as we can determine if the item was already in the cache based on the return from try_emplace.

[salman-javed-nz]

LGTM

clang-tools-extra/clang-tidy/GlobList.h

 class CachedGlobList final : public GlobList {
 public:
   using GlobList::GlobList;
 
   /// \see GlobList::contains
   bool contains(StringRef S) const override;
 
 private:
-  enum Tristate { None, Yes, No };
-  mutable llvm::StringMap<Tristate> Cache;
+  mutable llvm::StringMap<bool> Cache;
 };
 
 } // namespace tidy
 } // namespace clang
 
 #endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_GLOBLIST_H

clang-tools-extra/clang-tidy/GlobList.cpp

bool GlobList::contains(StringRef S) const {
   for (const GlobListItem &Item : llvm::reverse(Items)) {
     if (Item.Regex.match(S))
       return Item.IsPositive;
   }
   return false;
 }
 
 bool CachedGlobList::contains(StringRef S) const {
-  switch (auto &Result = Cache[S]) {
-  case Yes:
-    return true;
-  case No:
-    return false;
-  case None:
-    Result = GlobList::contains(S) ? Yes : No;
-    return Result == Yes;
-  }
-  llvm_unreachable("invalid enum");
+  auto Entry = Cache.try_emplace(S);
+  bool &Value = Entry.first->getValue();
+  // If the entry was just inserted, determine its required value.
+  if (Entry.second)
+    Value = GlobList::contains(S);
+  return Value;
 }
 
 } // namespace tidy
 } // namespace clang