A13-3-1: reconsider precision and feasibility of rule exception

[knewbury01]

the standard lists an exception where some explicit constructors may be constrained and therefore not actually overload implicit copy/move ctors.

the current implementation does not cover this,

further consideration is required to determine if the exception is possible and also valuable

consider the following :
not c.getTemplateArgument(_).toString().matches("%enable_if%") or maybe not c.getTemplateArgumentKind(_).toString().matches("%enable_if%")
but this wont work for functions where the types in the argument actually resolve if the function is called