dit pointed out on IRC that '__i = _VSTD::next(__i)' was a very long-winded...

dit pointed out on IRC that '__i = _VSTD::next(__i)' was a very long-winded way of writing '++__i'. Since I hate being thought of as long-winded (this checkin comment notwithstanding), I fixed it. No functionality change.

llvm-svn: 214834