Title: Always add title to LinkCache when necessary (in 3 methods)

Title: Always add title to LinkCache when necessary (in 3 methods)

Unless Title::GAID_FOR_UPDATE is specified, Title::getArticleID() will not
use the link cache if the Title already has the ID of the page. Account for
this by directly calling LinkCache::addLinkObj() in three other methods of
Title: getContentModel(), isRedirect(), and getLength().

Follows-up r33008 (aed9d4b91218) and dd5c1b7fb7ff, and reapplies part of
388b14a15de6.

Bug: T86974
Change-Id: I7eff0bffd4f632ceb8d2124af317d684dbcaf2cb