Move-assign from the specified rhs string and return *this.
Declared in <bslstl_string.h>
basic_string<char16_t>&
operator=(BloombergLP::bslmf::MovableRef<basic_string<char16_t>> rhs) noexcept(/* see-below */);
This function is potentially-throwing unless AllocatorTraits::propagate_on_container_move_assignment::value || AllocatorTraits::is_always_equal::value.