CWG2917 Disallow multiple friend-type-specifiers for a friend template

[jensmaurer]

The wording of P2893R3 (variadic friends), as adopted, does not properly cover template friends.

CWG suggests to exclude template friends from the scope of variadic friends for now, until such time a paper covers the issue.

See CWG2917 for details.

[jfbastien]

See #1567

[jensmaurer]

@jfbastien , you added to reference-to-self to this issue. Maybe that wasn't intentional?

[erichkeane]

EWG discussed this during the Monday AM session in Sofia.

The following poll was taken:

CWG2917: EWG consents to the reduction in scope for the variadic friends facility, as proposed in the resolution to CWG2917.

SF	F	N	A	SA
10	17	3	0	0

Result: Consensus