Add partial implementation of std::to_address() as llvm::to_address()

Summary:
Following on from the review for D58088, this patch provides the
prerequisite to_address() implementation that's needed to have
pointer_iterator support unique_ptr.

The late bound return should be removed once we move to C++14 to better
align with the C++20 declaration. Also, this implementation can be removed
once we move to C++20 where it's defined as std::to_addres()

The std::pointer_traits<>::to_address(p) variations of these overloads has
not been implemented.

Reviewers: dblaikie, paquette

Reviewed By: dblaikie

Subscribers: dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58421

llvm-svn: 354491