Rename languageServices(for:), primaryLanguageService(for:), and their
internal counterparts to use the `forOpenDocument` label, so the
precondition that the document must already be open is visible at call
sites.
Also make primaryLanguageService(forOpenDocument:) throw instead of
returning an optional, and switch several resolve-style handlers from
the find-or-create primaryLanguageService(for:_:) to
primaryLanguageService(forOpenDocument:), since those call sites already
have an open document in hand.
Rintaro Ishizaki
a39c92022f