[Clang][SVE2.1] Add clang support for builtins using svcount_t

In this patch it is used for the prototype:
  * svptrue_c8 (and _c16/_c32/_c64)

 As described in: https://github.com/ARM-software/acle/pull/257

Patch by: Sander de Smalen <sander.desmalen@arm.com>

Reviewed By: sdesmalen, david-arm

Differential Revision: https://reviews.llvm.org/D150953