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

[Shafik Yaghmour via cfe-commits]

================
@@ -9469,7 +9473,8 @@ static ExprResult buildCounterUpdate(
     if (!SemaRef.Context.hasSameType(Update.get()->getType(),
                                      VarRef.get()->getType())) {
       Update = SemaRef.PerformImplicitConversion(
-          Update.get(), VarRef.get()->getType(), Sema::AA_Converting, true);
+          Update.get(), VarRef.get()->getType(), AssignmentAction::Converting,
+          true);
----------------
shafik wrote:

```suggestion
          /*AllowExplicit=*/true);
```

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