[ADT] Rewrite mapped_iterator in terms of iterator_adaptor_base.

Summary:
This eliminates the boilerplate implementation of the iterator interface in
mapped_iterator.

This patch also adds unit tests that verify that the mapped function is applied
by operator* and operator->, and that references returned by the map function
are returned via operator*.

Reviewers: dblaikie, chandlerc

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D39855

llvm-svn: 317902