[clang] [C++20][Modules] Fix non-determinism in serialized AST (PR #110131)

[Ivan Murashko via cfe-commits]

https://github.com/ivanmurashko approved this pull request.

@eaeltsin confirmed that the commit fixes an issue introduced earlier by [#109167](https://github.com/llvm/llvm-project/pull/109167). The fix LGTM

https://github.com/llvm/llvm-project/pull/110131