Add next() and prior() iterator utility functions. Unlike std::advance

Add next() and prior() iterator utility functions. Unlike std::advance
they do not modify the passed iterator but return a copy.

next(myIt) returns copy of myIt incremented once
next(myIt, n) returns copy of myIt incremented n times
prior(myIt) returns copy of myIt decremented once
prior(myIt, n) returns copy of myIt decremented n times

While at it remove obsolete implementation of mapped_iterator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11429 91177308-0d34-0410-b5e6-96231b3b80d8