use std::optional to enforce ReadTimer

Author: alanzhao1

clang/include/clang/Serialization/ASTReader.h

@@ -526,6 +526,9 @@ class ASTReader
   /// A timer used to track the time spent deserializing.
   std::unique_ptr<llvm::Timer> ReadTimer;
 
+  // A TimeRegion used to start and stop ReadTimer via RAII.
+  std::optional<llvm::TimeRegion> ReadTimeRegion;
+
   /// The location where the module file will be considered as
   /// imported from. For non-module AST types it should be invalid.
   SourceLocation CurrentImportLoc;

clang/lib/Serialization/ASTReader.cpp

@@ -11004,8 +11004,8 @@ void ASTReader::diagnoseOdrViolations() {
 
 void ASTReader::StartedDeserializing() {
   if (llvm::Timer *T = ReadTimer.get();
-      ++NumCurrentElementsDeserializing == 1 && T && !T->isRunning())
-    T->startTimer();
+      ++NumCurrentElementsDeserializing == 1 && T)
+    ReadTimeRegion.emplace(T);
 }
 
 void ASTReader::FinishedDeserializing() {
@@ -11063,8 +11063,7 @@ void ASTReader::FinishedDeserializing() {
           (void)UndeducedFD->getMostRecentDecl();
       }
 
-      if (ReadTimer)
-        ReadTimer->stopTimer();
+      ReadTimeRegion.reset();
 
       diagnoseOdrViolations();
     }