Title::getTalkPage(): Restore behavior of interwiki-prefixed & fragment-only titles

NamespaceInfo::getTalkPage will throw an exception for these.
With this patch, Title::getTalkPage() will be reverted to the old
behavior of returning an incorrect meaningless value. Logging has been
added to identify code paths that trigger this behavior.

This patch should be undone once all such code paths have been found and
fixed.

Bug: T227817
Change-Id: I4727c7bb54d6f712ddcab05ef278a08d728f5726