[v3] Implement resolution of DR 630

[Paolo Carlini]

Jason Merrill wrote:
> Actually, yes, that's what the current draft says should happen. 
> Though it may not be what we want to happen.
Eh, interesting. Not knowing much about initializer_lists, I would have
wildly guessed either an empty list, with begin() == end() and the
assignment operator taking an initializer_list being selected anyway
(empty, thus the valarray becoming size == 0), or, a compile error....

Paolo.