[C++11] Replace llvm::next and llvm::prior with std::next and std::prev.

[C++11] Replace llvm::next and llvm::prior with std::next and std::prev.

Remove the old functions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202636 91177308-0d34-0410-b5e6-96231b3b80d8