[libc++] Implement deduction guides for <set>

This is part of C++17's P0433.

Thanks to Arthur O'Dwyer for the patch.

Differential Revision: https://reviews.llvm.org/D58582

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@363090 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed