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

[Dmitry Polukhin via cfe-commits]

dmpolukhin wrote:

@eaeltsin because I don't have a reproducer for the issue, could you please check that fix helps on your reproducer?

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