[libc++] Add C++17 deduction guides for std::function

Summary: http://llvm.org/PR39606

Reviewers: Quuxplusone

Subscribers: christof, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54410

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366484 91177308-0d34-0410-b5e6-96231b3b80d8