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

[Shafik Yaghmour via cfe-commits]

================
@@ -8819,8 +8822,8 @@ Expr *OpenMPIterationSpaceChecker::buildNumIterations(
           NewSize, Type->hasSignedIntegerRepresentation() ||
                        C.getTypeSize(Type) < NewSize);
       if (!SemaRef.Context.hasSameType(Diff.get()->getType(), NewType)) {
-        Diff = SemaRef.PerformImplicitConversion(Diff.get(), NewType,
-                                                 Sema::AA_Converting, true);
+        Diff = SemaRef.PerformImplicitConversion(
+            Diff.get(), NewType, AssignmentAction::Converting, true);
----------------
shafik wrote:

```suggestion
            Diff.get(), NewType, AssignmentAction::Converting, /*AllowExplicit=*/true);
```

nitpick

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