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

[Shafik Yaghmour via cfe-commits]

================
@@ -9491,8 +9496,8 @@ static ExprResult widenIterationCount(unsigned Bits, Expr *E, Sema &SemaRef) {
     return ExprResult(E);
   // OK to convert to signed, because new type has more bits than old.
   QualType NewType = C.getIntTypeForBitwidth(Bits, /*Signed=*/true);
-  return SemaRef.PerformImplicitConversion(E, NewType, Sema::AA_Converting,
-                                           true);
+  return SemaRef.PerformImplicitConversion(E, NewType,
+                                           AssignmentAction::Converting, true);
----------------
shafik wrote:

```suggestion
                                           AssignmentAction::Converting, /*AllowExplicit=*/true);
```

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