Requiring C++20 Modules Serialization Tests for New Features

[cor3ntin]

RFC: https://discourse.llvm.org/t/rfc-clang-requiring-c-20-modules-serialization-tests-for-new-features/90768/12

[AaronBallman]

This was discussed at the Clang Area Team meeting on May 28: https://docs.google.com/document/d/13-68BCOt8kH5k_56fURBi7u7Ap97Z0EshgkJXDudbJM/edit?pli=1&tab=t.0

Aaron posted a follow-up comment to the RFC: https://discourse.llvm.org/t/rfc-clang-requiring-c-20-modules-serialization-tests-for-new-features/90768/15