[IR] Add LLVM IR support for target("aarch64.svcount") type.

The C and C++ Language Extensions for AArch64 SME2 [1] 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
opaque type.

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.

[1] https://github.com/ARM-software/acle/pull/217

Reviewed By: jcranmer-intel, nikic

Differential Revision: https://reviews.llvm.org/D136861