Fixed few remarks mentioned by @Quuxplusone.

Thanks a lot @Quuxplusone! I am aware that you cannot take any final decision on what gets in but these are really helpful comments here.
I will fix some of you remarks and upload a better diff (with proper context to get a better picture) afterwards.

Removed the unnecessary variables __sentinel.

Note: this is my first contribution to libc++. I tried to follow the style used in the rest of files, but I may have missed some hidden coding style rules: some files use four spaces, some two? Which SFINAE pattern (return type, class paramter...) is to be preferred? Free template functions vs template classes with static member functions?