[Serialization] Use llvm::is_contained (NFC)

Author: kazutakahirata

clang/lib/Serialization/ASTReader.cpp

@@ -10115,8 +10115,7 @@ void ASTReader::pushExternalDeclIntoScope(NamedDecl *D, DeclarationName Name) {
     // Adding the decl to IdResolver may have failed because it was already in
     // (even though it was not added in scope). If it is already in, make sure
     // it gets in the scope as well.
-    if (std::find(SemaObj->IdResolver.begin(Name),
-                  SemaObj->IdResolver.end(), D) != SemaObj->IdResolver.end())
+    if (llvm::is_contained(SemaObj->IdResolver.decls(Name), D))
       SemaObj->TUScope->AddDecl(D);
   }
 }