Allow some template function instantiation

[arnetheduck]

There are several places where instantiating templates for specific types would be useful - two obvious examples are QVariant::fromValue and QMetaType::qMetaTypeId where the obvious instantiations are for QObject*, void* and so on (so you can create variants that hold qobject and/or reason about metatypes) - a general mechanism for enumerating such instantiations would be useful.