[libcxx] Fix PR24075, PR23841 - Add...

[libcxx] Fix PR24075, PR23841 - Add scoped_allocator_adaptor::construct(pair<T, U>*, ...) overloads.

Summary:
For more information see:

* https://llvm.org/bugs/show_bug.cgi?id=23841
* https://llvm.org/bugs/show_bug.cgi?id=24075

I hope you have as much fun reviewing as I did writing these insane tests!

Reviewers: mclow.lists, AlisdairM, EricWF

Subscribers: AlisdairM, Potatoswatter, cfe-commits

Differential Revision: https://reviews.llvm.org/D27612

llvm-svn: 289710