[AST] Initialize local counter

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.

Reviewed By: bnbarham, barannikov88

Differential Revision: https://reviews.llvm.org/D150492

clang/lib/Frontend/ASTUnit.cpp

@@ -822,7 +822,6 @@ std::unique_ptr<ASTUnit> ASTUnit::LoadFromASTFile(
   // 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,
@@ -846,6 +845,7 @@ std::unique_ptr<ASTUnit> ASTUnit::LoadFromASTFile(
       /*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));