[SveEmitter] Add builtins for svwhile

This also adds the IsOverloadWhile flag which tells CGBuiltin to use
both the default type (predicate) and the type of the second operand
(scalar) as the overloaded types for the LLMV IR intrinsic.

Reviewers: SjoerdMeijer, efriedma, rovka

Reviewed By: efriedma

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77595