Tested on Linux-PPC64.

2015-07-21  Ville Voutilainen  <ville.voutilainen@gmail.com>
    Implement N4279, Improved insertion interface for unique-key maps.
    * include/bits/stl_map.h (try_emplace, insert_or_assign): New.
    * include/bits/stl_tree.h (_M_get_insert_unique_pos,
    _M_get_insert_equal_pos, _M_get_insert_hint_unique_pos,
    _M_get_insert_hint_equal_pos): Make public.
    * include/bits/unordered_map.h (try_emplace, insert_or_assign): New.
    * testsuite/23_containers/map/modifiers/insert_or_assign/1.cc:
    Likewise.
    * testsuite/23_containers/map/modifiers/try_emplace/1.cc: Likewise.
    * testsuite/23_containers/unordered_map/modifiers/insert_or_assign.cc:
    Likewise.
    * testsuite/23_containers/unordered_map/modifiers/try_emplace.cc:
    Likewise.