Remove no-op callbacks from TemplateInstantiationCallback

[gribozavr]

Summary

It is not clear to me that these callbacks ever did anything, they were
empty even in the original implementation (r324808).

[xazax.hun]

The changes look good to me. Added some folks who might have more knowledge about the original purpose.

[Szelethus]

They are used by Templight, which we are planning to eventually move to clang-tools extra, though our resources are a bit limited at the time :)

https://github.com/mikael-s-persson/templight/blob/master/include/TemplightTracer.h
https://github.com/mikael-s-persson/templight/blob/master/lib/TemplightTracer.cpp#L306

clang/include/clang/Sema/TemplateInstCallback.h

 namespace clang {
 
 /// This is a base class for callbacks that will be notified at every
 /// template instantiation.
 class TemplateInstantiationCallback {
 public:
   virtual ~TemplateInstantiationCallback() = default;
 
-  /// Called before doing AST-parsing.
-  virtual void initialize(const Sema &TheSema) = 0;
-
-  /// Called after AST-parsing is completed.
-  virtual void finalize(const Sema &TheSema) = 0;
-
   /// Called when instantiation of a template just began.
   virtual void atTemplateBegin(const Sema &TheSema,
                                const Sema::CodeSynthesisContext &Inst) = 0;
 
   /// Called when instantiation of a template is just about to end.
   virtual void atTemplateEnd(const Sema &TheSema,
                              const Sema::CodeSynthesisContext &Inst) = 0;
 };
 
 template <class TemplateInstantiationCallbackPtrs>
-void initialize(TemplateInstantiationCallbackPtrs &Callbacks,
-                const Sema &TheSema) {
-  for (auto &C : Callbacks) {
-    if (C)
-      C->initialize(TheSema);
-  }
-}
-
-template <class TemplateInstantiationCallbackPtrs>
-void finalize(TemplateInstantiationCallbackPtrs &Callbacks,
-              const Sema &TheSema) {
-  for (auto &C : Callbacks) {
-    if (C)
-      C->finalize(TheSema);
-  }
-}
-
-template <class TemplateInstantiationCallbackPtrs>
 void atTemplateBegin(TemplateInstantiationCallbackPtrs &Callbacks,
                      const Sema &TheSema,
                      const Sema::CodeSynthesisContext &Inst) {
   for (auto &C : Callbacks) {
     if (C)
       C->atTemplateBegin(TheSema, Inst);
   }
 }

clang/lib/Frontend/FrontendActions.cpp

 } // namespace yaml
 } // namespace llvm
 
 namespace {
 class DefaultTemplateInstCallback : public TemplateInstantiationCallback {
   using CodeSynthesisContext = Sema::CodeSynthesisContext;
 
 public:
-  void initialize(const Sema &) override {}
-
-  void finalize(const Sema &) override {}
-
   void atTemplateBegin(const Sema &TheSema,
                        const CodeSynthesisContext &Inst) override {
     displayTemplightEntry<true>(llvm::outs(), TheSema, Inst);
   }
 
   void atTemplateEnd(const Sema &TheSema,
                      const CodeSynthesisContext &Inst) override {
     displayTemplightEntry<false>(llvm::outs(), TheSema, Inst);

clang/lib/Parse/ParseAST.cpp

if (PrintStats) {
     Decl::EnableStatistics();
     Stmt::EnableStatistics();
   }
 
   // Also turn on collection of stats inside of the Sema object.
   bool OldCollectStats = PrintStats;
   std::swap(OldCollectStats, S.CollectStats);
 
-  // Initialize the template instantiation observer chain.
-  // FIXME: See note on "finalize" below.
-  initialize(S.TemplateInstCallbacks, S);
-
   ASTConsumer *Consumer = &S.getASTConsumer();
 
   std::unique_ptr<Parser> ParseOP(
       new Parser(S.getPreprocessor(), S, SkipFunctionBodies));
   Parser &P = *ParseOP.get();
 
   llvm::CrashRecoveryContextCleanupRegistrar<const void, ResetStackCleanup>
       CleanupPrettyStack(llvm::SavePrettyStackState());
if (HaveLexer) {
     }
   }
 
   // Process any TopLevelDecls generated by #pragma weak.
   for (Decl *D : S.WeakTopLevelDecls())
     Consumer->HandleTopLevelDecl(DeclGroupRef(D));
 
   Consumer->HandleTranslationUnit(S.getASTContext());
-
-  // Finalize the template instantiation observer chain.
-  // FIXME: This (and init.) should be done in the Sema class, but because
-  // Sema does not have a reliable "Finalize" function (it has a
-  // destructor, but it is not guaranteed to be called ("-disable-free")).
-  // So, do the initialization above and do the finalization here:
-  finalize(S.TemplateInstCallbacks, S);
-
   std::swap(OldCollectStats, S.CollectStats);
   if (PrintStats) {
     llvm::errs() << "\nSTATISTICS:\n";
     if (HaveLexer) P.getActions().PrintStats();
     S.getASTContext().PrintStats();
     Decl::PrintStats();
     Stmt::PrintStats();
     Consumer->PrintStats();
   }
 }