[ntuple] Wrong alignment with transient class member

[hahnjo]

Description

RClassField::Attach only takes into account persistent members to determine alignment:

root/tree/ntuple/v7/src/RField.cxx

Lines 1874 to 1879 in 3091720

	void ROOT::Experimental::RClassField::Attach(std::unique_ptr<RFieldBase> child, RSubFieldInfo info)
	{
	fMaxAlignment = std::max(fMaxAlignment, child->GetAlignment());
	fSubFieldsInfo.push_back(info);
	RFieldBase::Attach(std::move(child));
	}

Reproducer

#include <ROOT/RField.hxx>

class ClassWithTransient {
  char a;
  int t; //!
  char b;
};

void ntuple_class_align() {
  ROOT::Experimental::RField<ClassWithTransient> f("f");
  std::cout << "value size: " << f.GetValueSize() << ", alignment: " << f.GetAlignment() << "\n";
}

output:

Processing ntuple_class_align.C...
value size: 12, alignment: 1

This can lead to problems if such class is embedded into a record, for example a std::pair.

[hahnjo]

#21669 provides TClass::GetClassAlignment(), which we should use in RClassField and RStreamerField

[jblomer]

Backport to 6.40: #22576