[SveEmitter] Add builtins for svwhilerw/svwhilewr

This also adds the IsOverloadWhileRW flag which tells CGBuiltin to use
the result predicate type and the first pointer type as the
overloaded types for the LLVM IR intrinsic.

Reviewers: SjoerdMeijer, efriedma

Reviewed By: efriedma

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78238