polymorphic_allocator() noexcept;
Effects: Sets memory_rsrc to get_default_resource().
polymorphic_allocator(memory_resource* r);
Requires: r is non-null.
Effects: Sets memory_rsrc to r.
Throws: Nothing.
Notes: This constructor provides an implicit conversion from memory_resource*.
template <class U>
polymorphic_allocator(const polymorphic_allocator<U>& other) noexcept;
Effects: Sets memory_rsrc to other.resource().