The C and C++ Language Extensions for AArch64 SME2  adds a new type called
svcount_t which describes a predicate. This is not a predicate vector
mask, but rather a description of a predicate vector mask that can be
expanded into a mask using explicit instructions. The type is a scalable
To implement svcount_t type this patch uses the existing Target Extension Type
mechanism, but adds further support so that this type can be a scalable type.
AArch64 CodeGen support will follow in a separate patch.