1. Fix make_shared<const T>. 2. Allow allocator<const T> as an extension. ...

1.  Fix make_shared<const T>.  2.  Allow allocator<const T> as an extension.  3.  Refactor work which fixed unique_ptr<const T[]>.  4.  Remove no-longer-needed private declarations from unique_ptr.  5.  Add constraints to some shared_ptr and weak_ptr constructors and assignment operators so that is_constructible/is_assignable give the correct answers for shared_ptr and weak_ptr.  6.  Make defensive preparations in the shared_ptr free functions for the introduction of shared_ptr<T[]> in the future.  7.  As an optimization, add move constructor and move assignment to weak_ptr.

llvm-svn: 147437