[PATCH] Add support for "fancy" pointers to shared_ptr. Fixes PR20616

[Eric Fiselier]

Hi mclow.lists, danalbert,

This patch add support for "fancy pointers/allocators" as well as fixing support for shared_pointer and "minimal" allocators.

Fancy pointers are class types that meet the NullablePointer requirements. In our case they are created by fancy allocators. `support/min_allocator.h` is an archetype for these types.

There are three types of changes made in this patch:
1. `_Alloc::template rebind<T>::other` -> `__allocator_traits_rebind<_Alloc, T>::type`.
This change was made because allocators don't need a rebind template. `__allocator_traits_rebind` is used instead of `allocator_traits::rebind` because use of `allocator_traits::rebind` requires a workaround for when template aliases are unavailable.
2. `a.deallocate(this, 1)` -> `a.deallocate(pointer_traits<self>::pointer_to(*this), 1)`. This change change is made because fancy pointers aren't always constructible from raw pointers. 
3. `p.get()` -> `addressof(*p.get())`. Fancy pointers aren't actually a pointer. When we need a "real" pointer we take the address of dereferencing the fancy pointer. This should give us the actual raw pointer.

http://reviews.llvm.org/D4859

Files:
  include/memory
  test/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.const/nullptr_t_deleter_allocator.pass.cpp
  test/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.const/pointer_deleter_allocator.pass.cpp
  test/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.create/allocate_shared.pass.cpp
  test/utilities/memory/util.smartptr/util.smartptr.shared/util.smartptr.shared.create/allocate_shared_no_variadics.pass.cpp
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