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 adds an opaque LLVM IR type as well as
support for parsing, assembling and a LangRef description. AArch64 CodeGen
support will follow in a separate patch.
Patch co-authored by @CarolineConcatto