LocalisationCache: try harder to use LCStoreCDB

When the LocalisationCache class is set to 'detect', we prefer LCStoreCDB if
$wgCacheDirectory is set, but we default to LCStoreDB otherwise. Rather than
give up, we should try wfTempDir(), since any directory that meets its criteria
is also suitable for LCStoreCDB.

Change-Id: Id3e2d2b18ddb423647bf2e893bcf942722c0e097