Fix a minor bug with std::next and prev not and negative numbers. In particular, std::prev cannot require Bidirectional Iterators, because you might 'go back' -1 places, which goes forward. Thanks to Ville and Jonathan for the bug report.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356818 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed