Implicit copy constructor for virtual bases

[Jason Merrill]

A correction: This behavior is unspecified, not undefined.  "undefined" has
specific meaning which does not apply here.  Also, "when being copied"
should be clarified to say assignment, since the copy constructor also
copies objects.

Saying "should be provided" is a bit strong; I would expect that most code
wouldn't really care how often virtual bases are copy-assigned.

Jason