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

Thanks to Arthur O'Dwyer for the patch.
Differential Revision: https://reviews.llvm.org/D58617

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365788 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed