Fix if-feature propagation for implicit case statements

Implicit statements created during parse must not be propagated
if their child statement is not supported by features or otherwise
unavailable.

This really can only happen when we are creating an implicit declaration
on the parser side, as in other code paths the substatement is already
completely defined.

JIRA: YANGTOOLS-1431
Change-Id: I5797f09c795587c91f10c7f92a6a8c9fd2480213
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>