* generic.cs
(ConstraintChecker.HasDefaultConstructor): If we're a TypeBuilder
and don't have any instance constructors, also lookup in the base class.
(TypeManager.IsNullableValueType): New public method.
* typemanager.cs (TypeManager.MemberLookup_FindMembers): Clear the
`BindingFlags.DeclaredOnly' flag and set `used_cache'.
(TypeManager.TryGetBaseDefinition): Use DropGenericMethodArguments().
* expression.cs (Unary.DoResolve): Use TypeManager.IsNullableValueType()
instead of just TypeManager.IsNullableType() to determine whether
a lifted operator exists.
(UnaryMutator.DoResolve): Likewise.
(Conditional.DoResolve): Likewise.
(Binary.DoResolve): A lifted operator only exists if both operands
are valuetypes and at least one of them is a nullable type.
svn path=/trunk/mcs/; revision=58071
