(bug 40340) Fix cache issues with changing user groups

(bug 40340) Fix cache issues with changing user groups

* migrateUserGroup.php: Call User::invalidateCache

* While at it, also fix the issue where User::clearInstanceCache
  did not clear cache for User::getGroups.

  Although it does clear the caches of methods used to calculate
  other group-related lists (such as User::getEffectiveGroups),
  the one for the query from user_groups was still cached in
  $this->mGroups.

  Presumably this was forgotten when this pattern was introduced
  as the instance cache precedes the user_group table.

Change-Id: I22abdba00f8ccf587a3d7696e57970ed4653afc8