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

This commit adds a node handle type, (located in __node_handle), and adds
extract() and insert() members to all map and set types, as well as their
implementations in __tree and __hash_table.

The second half of this feature is adding merge() members, which splice nodes
in bulk from one container into another. This will be committed in a follow-up.

Differential revision: https://reviews.llvm.org/D46845

llvm-svn: 338472