[Serialization] Fix a warning

This patch fixes:

  clang/lib/Serialization/ASTReader.cpp:9978:27: error: lambda capture
  'this' is not used [-Werror,-Wunused-lambda-capture]

Author: kazutakahirata

clang/lib/Serialization/ASTReader.cpp

@@ -9975,7 +9975,7 @@ void ASTReader::finishPendingActions() {
       return false;
     };
 
-    auto hasDefinition = [this, &hasDefinitionImpl](Decl *D) {
+    auto hasDefinition = [&hasDefinitionImpl](Decl *D) {
       return hasDefinitionImpl(D, hasDefinitionImpl);
     };