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

[Dan Liew via cfe-commits]

delcypher wrote:

@Endilll This is my attempt at implementing what you asked for in https://github.com/llvm/llvm-project/pull/106321#discussion_r1734379624

I'm not a huge fan of this as I think this will be extremely annoying for downstream forks with very little upside vs just moving the unscoped enum to be earlier in the `Sema` class declaration (which creates no churn).

However, if you and @AaronBallman want this version of the PR then we can with this.

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