[libc++] Fix bug in allocate_shared_for_overwrite

Instead of destroying the object with allocator::destroy, we must
call its destructor directly. As a fly-by also mark LWG3008 as
fixed since it is handled by our implementation.

This was pointed out by Tim Song in https://reviews.llvm.org/D140913.

Differential Revision: https://reviews.llvm.org/D143791