Fix StatementContextBase.childCopyOf()
Most of our StmtContext implementations are derived from
StatementContextBase, but notably ReplicaStatementContext is not -- and
we do not handle it during copy operations.
Update the dispatch code to short circuit to replicaAsChildOf(), which
takes care of the details.
JIRA: YANGTOOLS-1346
Change-Id: I65f9713d5c06bbe251a4b27fb0745dd905c36976
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
3b4308a5a4988fb66839c8d2724c13e0ae1b2e9c)