The TClass generated by TStreamerInfo::GenerateInfoForPair is neither 'Interpreted' (cling
likely does not, yet, have any information about that pair) nor loaded (the user did not
request a dictionary for it) but need to have special treatment for those TClass.

It is not clear whether this should be a new state.  For now we introduce a new
data member (`fIsSyntheticPair`) and don't increase the size of TClass instance
by using a bit field.

…re provided.

Since getting the right alignment and padding is hard (either use Cling/Clang with the associated memory cost and potential
autoparsing or duplicating the platform dependent code that calculates it), we now only creates the synthetic TClass instance
that represent and std::pair ***if and only*** the call is provided the actual offset of second and sizeof the pair.

This information is known to compiled (and later interpreted) CollectionProxy, to TClass for class containing an std::pair
(via their list of RealData which is recorded in the rootpcm) and to TClass::GetClass templated on the actual type
(since the pair's data members are public)

Extract the offset and size of the pair and pass it along to raw TClass::GetClass

Use the data in fPRealData to calculate the offset and size of the pair

Note: If a StreamerInfo is loaded from a file and is the same information
as an existing one, it is assigned the same "unique id" and we need to double
check before removing it from the global list.

…air.