Open too many different non-versioned layouts for pair

[pcanal]

As reported originally at: cms-sw/cmssw#37113, TClass/TStreamerInfo can complain that they are too many StreamerInfo for an std::class.

[pcanal]

The core issue is that TDataMember::Init and TStreamerInfo::GenerateInfoForPair were not consistent. TDataMember::Init was ignoring the underlying type of an enum while the newer TStreamerInfo::GenerateInfoForPair was taking it in consideration. In the reported case, it meant that some of the pair's TStreamerInfo recorded the type as being 'signed intwhile other was recording the type asunsigned int`.

In addition the whole infrastructure assumed (but only in "some/most" places) that the TStreamerInfo for a std::pair could never change and the infrastructure was also inconsistent on knowing whether the schema layout for std::pair is version of non-versioned.