AbstractCodeGenerationService.AddMembers was using an O(n^2) algorithm to insert new members.  This change allows bulk inserting members if the AutoInsertionLocation flag is false.  For now, metadata as source only sets AutoInsertionLocation to false for enum symbols which can commonly have thousands of members.

SeparatedSyntaxList.InsertRange was also calling Insert for each node.  It now adds the nodes in a single step. (changeset 1344073)