[clang] [clang] Fix assertion failure in constexpr union deserialization (PR #140179)

[Alexander Kornienko via cfe-commits]

alexfh wrote:

> I think we need a release note in clang/docs/ReleaseNotes.rst

Which aspect would you like to cover in the release notes? The fix of the mismatch between AST writer and AST reader or something else? Do you have a specific wording in mind?

https://github.com/llvm/llvm-project/pull/140179