[mlir][Types] Remove the subclass data from Type

Subclass data is useful when a certain amount of memory is allocated, but not all of it is used. In the case of Type, that hasn't been the case for a while and the subclass is just taking up a full `unsigned`. Removing this frees up ~8 bytes for almost every type instance.

Differential Revision: https://reviews.llvm.org/D85348