Make Language::hasVariant() more strict

Make Language::hasVariant() more strict

In d59f27aeab08b171e5ab6a081e763a4cad0bca04 we made
LanguageConverter::validateVariant() try harder to convert a variant
into an acceptable MediaWiki-internal form, looking at deprecated
codes and BCP 47 aliases.  However, this misled Language::hasVariant()
into thinking that bogus names (like all-uppercase strings) were
acceptable variant names, which then led exceptions when they were
passed to the various conversion methods.

This is a belt-and-suspenders patch for T207433 -- in that case we
shouldn't have created a Language object with code 'sr-cyrl' in the
first place, but once one was created we shouldn't have tried to
ask LanguageSr to convert texts to 'sr-cyrl'.  The latter problem
is fixed by this patch.

Bug: T207433
Change-Id: Id993bc7989144b5031a551662e8e492bd23f698a