[SveEmitter] Add IsOverloadNone flag and builtins for svpfalse and svcnt[bhwd]_pat

Add the IsOverloadNone flag to tell CGBuiltin that it does not have
an overloaded type. This is used for e.g. svpfalse which does
not take any arguments and always returns a svbool_t.

This patch also adds builtins for svcntb_pat, svcnth_pat, svcntw_pat
and svcntd_pat, as those don't require custom codegen.

Reviewers: SjoerdMeijer, efriedma, rovka

Reviewed By: efriedma

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77596