[ntuple] defriend RClassField from RFieldBase

The privileged access of RClassField to RFieldBase is used to modify
some properties of its subfields. Make this more targeted through
protected methods.

One of the methods, CallSetArtificialOn(), will be reused by the record
field.

Author: jblomer

tree/ntuple/inc/ROOT/RFieldBase.hxx

@@ -85,7 +85,6 @@ This is and can only be partially enforced through C++.
 */
 // clang-format on
 class RFieldBase {
-   friend class ROOT::RClassField;                             // to mark members as artificial
    friend class ROOT::Experimental::Detail::RRawPtrWriteEntry; // to call Append()
    friend struct ROOT::Internal::RFieldCallbackInjector;       // used for unit tests
    friend struct ROOT::Internal::RFieldRepresentationModifier; // used for unit tests
@@ -421,6 +420,11 @@ protected:
    static void CallConstructValueOn(const RFieldBase &other, void *where) { other.ConstructValue(where); }
    static std::unique_ptr<RDeleter> GetDeleterOf(const RFieldBase &other) { return other.GetDeleter(); }
 
+   /// Allow parents to mark their childs as artificial fields (used in class and record fields)
+   void CallSetArtificialOn(RFieldBase &other) { other.SetArtificial(); }
+   /// Allow class fields to adjust the type alias of their members
+   void SetTypeAliasOf(RFieldBase &other, const std::string &alias) { other.fTypeAlias = alias; }
+
    /// Operations on values of complex types, e.g. ones that involve multiple columns or for which no direct
    /// column type exists.
    virtual std::size_t AppendImpl(const void *from);

tree/ntuple/src/RFieldMeta.cxx

@@ -174,9 +174,9 @@ ROOT::RClassField::RClassField(std::string_view fieldName, TClass *classp)
 
       const auto normTypeName = ROOT::Internal::GetNormalizedUnresolvedTypeName(origTypeName);
       if (normTypeName == subField->GetTypeName()) {
-         subField->fTypeAlias = "";
+         SetTypeAliasOf(*subField, "");
       } else {
-         subField->fTypeAlias = normTypeName;
+         SetTypeAliasOf(*subField, normTypeName);
       }
 
       fTraits &= subField->GetTraits();
@@ -190,7 +190,7 @@ ROOT::RClassField::~RClassField()
    if (fStagingArea) {
       for (const auto &[_, si] : fStagingItems) {
          if (!(si.fField->GetTraits() & kTraitTriviallyDestructible)) {
-            auto deleter = si.fField->GetDeleter();
+            auto deleter = GetDeleterOf(*si.fField);
             deleter->operator()(fStagingArea.get() + si.fOffset, true /* dtorOnly */);
          }
       }
@@ -467,7 +467,7 @@ void ROOT::RClassField::BeforeConnectPageSource(ROOT::Internal::RPageSource &pag
    // Iterate over all sub fields in memory and mark those as missing that are not in the descriptor.
    for (auto &field : fSubfields) {
       if (regularSubfields.count(field->GetFieldName()) == 0) {
-         field->SetArtificial();
+         CallSetArtificialOn(*field);
       }
    }
 }