[AST] Initialize local counter

[vitalybuka]

Summary

I assume it's optional and ReadAST does not have to set the
counter on success.

Without the patch msan complains that we pass
uninitialized value into noundef parameters of setCounterValue.

[vitalybuka]

ping

[barannikov88]

Not my area, but the change is trivial.

Nit: could move the variable closer to its use.

[bnbarham]

clang/lib/Frontend/ASTUnit.cpp

825

I assume it's optional and ReadAST does not have to set the counter on success.

Looks like we only write the counter value when it's non-zero. So the reader would never set it. Thanks for fixing!

clang/lib/Frontend/ASTUnit.cpp

AST->HeaderInfo.reset(new HeaderSearch(AST->HSOpts,
                                          AST->getDiagnostics(),
                                          AST->getLangOpts(),
                                          /*Target=*/nullptr));
   AST->PPOpts = std::make_shared<PreprocessorOptions>();
 
   // Gather Info for preprocessor construction later on.
 
   HeaderSearch &HeaderInfo = *AST->HeaderInfo;
-  unsigned Counter;
 
   AST->PP = std::make_shared<Preprocessor>(
       AST->PPOpts, AST->getDiagnostics(), *AST->LangOpts,
       AST->getSourceManager(), HeaderInfo, AST->ModuleLoader,
       /*IILookup=*/nullptr,
       /*OwnsHeaderSearch=*/false);
   Preprocessor &PP = *AST->PP;
 
   if (ToLoad >= LoadASTOnly)
     AST->Ctx = new ASTContext(*AST->LangOpts, AST->getSourceManager(),
                               PP.getIdentifierTable(), PP.getSelectorTable(),
                               PP.getBuiltinInfo(),
                               AST->getTranslationUnitKind());
 
   DisableValidationForModuleKind disableValid =
       DisableValidationForModuleKind::None;
   if (::getenv("LIBCLANG_DISABLE_PCH_VALIDATION"))
     disableValid = DisableValidationForModuleKind::All;
   AST->Reader = new ASTReader(
       PP, *AST->ModuleCache, AST->Ctx.get(), PCHContainerRdr, {},
       /*isysroot=*/"",
       /*DisableValidationKind=*/disableValid, AllowASTWithCompilerErrors);
 
+  unsigned Counter = 0;
   AST->Reader->setListener(std::make_unique<ASTInfoCollector>(
       *AST->PP, AST->Ctx.get(), *AST->HSOpts, *AST->PPOpts, *AST->LangOpts,
       AST->TargetOpts, AST->Target, Counter));
 
   // Attach the AST reader to the AST context as an external AST
   // source, so that declarations will be deserialized from the
   // AST file as needed.
   // We need the external source to be set up before we read the AST, because