Improve SBE stage builder to not require kResult in buildSimpleProjection and buildSort:

• fix buildSimpleProjection to not pass down kResult
• fix buildSort to not eagerly require kResult and instead use kResult or fields from output as returned from the child. And in addition to that, if child decided to return kResult in output despite not being asked to do so, discard it before buffering. Or assert that it's impossible