Rework some bits in Parallel LINQ to work under Full AOT.

mcs/class/System.Core/System.Linq.Parallel.QueryNodes/QueryGroupByNode.cs

@@ -69,7 +69,7 @@ namespace System.Linq.Parallel.QueryNodes
 
 		internal IEnumerable<IGrouping<TKey, TElement>> GetGroupedElements ()
 		{
-			return GetStore ().Select (e => (IGrouping<TKey,TElement>)new ConcurrentGrouping<TKey, TElement> (e.Key, e.Value));
+			return (IEnumerable<System.Linq.IGrouping<TKey,TElement>>)GetStore ().Select (e => (IGrouping<TKey,TElement>)new ConcurrentGrouping<TKey, TElement> (e.Key, e.Value));
 		}
 
 		internal ConcurrentDictionary<TKey, ConcurrentQueue<TElement>> GetStore ()

mcs/class/System.Core/System.Linq.Parallel/ConcurrentLookup.cs

@@ -109,7 +109,7 @@ namespace System.Linq.Parallel
 
 		IEnumerator<IGrouping<TKey, TElement>> GetEnumeratorInternal ()
 		{
-			return dictionary.Select ((pair) => new ConcurrentGrouping<TKey, TElement> (pair.Key, pair.Value)).GetEnumerator ();
+			return (IEnumerator<System.Linq.IGrouping<TKey,TElement>>) dictionary.Select ((pair) => new ConcurrentGrouping<TKey, TElement> (pair.Key, pair.Value)).GetEnumerator ();
 		}
 	}
 }