Fix leafref require-instance implementation

Fix leafref require-instance implementation

Both identityref and leafref types default to require-instace=true,
hence we should be starting with that and also properly inherit
the property from the parent type.

Add RequireInstanceRestrictedTypeDefinition to capture the common
method and adjust RequireInstanceRestrictedTypeBuilder to check
for it and initialize the require-instance value appropriately.

JIRA: YANGTOOLS-872
Change-Id: I125d5687950af21694c4510ce64adb00f92ae4e6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>