Fix #76417: a generic class may now have methods which may unify
for some type parameter substitutions.
* class.cs (Method.IsDuplicateImplementation): Don't report CS0408
for methods which may unify anymore.
* expression.cs (Invocation.MoreSpecific): New private static
method; checks whether one method is more specific than another
according to 14.4.2.2 of the spec.
(Invocation.BetterFunction): Implement the tie-breaking rules from
14.4.2.2 of the spec: if two methods unify for some type parameter
substitution, we need to pick the more specific one.
svn path=/trunk/mcs/; revision=55728
