[libcxx] Re-commit: shared_ptr changes from library fundamentals (P0414R2).

Implements P0414R2:
  * Adds support for array types in std::shared_ptr.
  * Adds reinterpret_pointer_cast for shared_ptr.

Re-committing now that the leaking tests are fixed.

Differential Revision: https://reviews.llvm.org/D62259