LRM93: useless restrictions on signal attributes

The signal attributes of kind function (event, active, ...) require a static prefix name. This is a useless restriction as these attributes are available from a function.

So you can always write:

   function has_event(signal s : bit) return boolean is
   begin
     return s'event;
   end has_event;

   ... has_event (bv(i)) ...

if you want to write:

   ... bv(i)'event ...

Proposed solution:
remove these restrictions.