LWG2070: `allocate_shared` should use `allocator_traits<A>::construct`

[ldionne]

Link: https://wg21.link/LWG2070

BEGIN-RST-NOTES
Resolved by `P0674R1 <https://wg21.link/P0674R1>`__
END-RST-NOTES

[ldionne]

This issue has already been implemented and was created for tracking purposes only.

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Louis Dionne (ldionne)

Details

**Link:** https://wg21.link/LWG2070