Fix C++17 Xcode build when the new architecture is enabled

[christophpurrer]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

These changes were 'missed' in #1498

Changelog

[macOS] [Fixed] - Fix C++17 Xcode build when the new architecture is enabled

Test Plan

Build app with Fabric enabled and run it:

yarn
cd packages/rn-tester
bundle install && USE_FABRIC=1 bundle exec pod install
open RNTesterPods.xcworkspace

[Saadnajmi]

/azp run

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