[clang] [NFC][Sema] Move `Sema::AssignmentAction` into its own scoped enum (PR #106453)

[Vlad Serebrennikov via cfe-commits]

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

Thank you for resolving your issue the hard way, and paying some of the technical debt we accumulated over the years.

https://github.com/llvm/llvm-project/pull/106453