Second half of C++17's splicing maps and sets

This commit adds a merge member function to all the map and set containers,
which splices nodes from the source container. This completes support for
P0083r3.

Differential revision: https://reviews.llvm.org/D48896

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345744 91177308-0d34-0410-b5e6-96231b3b80d8