[libc++] Fix allocate_shared when used with an explicitly convertible allocator

[libc++] Fix allocate_shared when used with an explicitly convertible allocator

When the allocator is only explicitly convertible from other specializations
of itself, the new version of std::allocate_shared would not work because
it would try to do an implicit conversion. This patch fixes the problem
and adds a test so that we don't fall into the same trap in the future.