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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@214834 91177308-0d34-0410-b5e6-96231b3b80d8