[Clang] Create opaque type for AArch64 SVE2p1/SME2 svcount_t.

This patch adds the builtin type __SVCount_t to Clang, which is an opaque
scalable type defined in the SME2 C and C++ Language Extensions.

The type maps to the `target("aarch64.svcount")` LLVM IR type.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D136864