Implement proposed resolution for LWG#2758. Reviewed as D24446. Normally, I...

Implement proposed resolution for LWG#2758. Reviewed as D24446. Normally, I would wait for these to be voted upon at a committee meeting (November), but the current draft standard is broken, and this should fix it. (And if it doesn't, we want to know about it soonest)

llvm-svn: 282342