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.

llvm-svn: 11429