Don't complain when the same type is implemented by the

Don't complain when the same type is implemented by the
output assembly as well as multiple referenced assemblies

* namespace.cs (RootNamespace.LookupTypeReflection): Add
'must_be_unique' flag.
(GlobalRootNamespace): Update to changes.
(Namespace.LookupType): Pass 'must_be_unique' only when we don't
already have a type in hand.

svn path=/trunk/mcs/; revision=138385