Remove support for `-language:_`

[nicolasstucki]

We should just remove support for -language:_, it doesn't mean anything useful. But even then, I think it's strange that some -language flags just turn off warnings whereas others change the semantics of the language (and are therefore much more dangerous), I think we should clearly separate these two sets of options, in particular turning off language warnings should look like turning off any other kind of warning (something like the -Wno-xxx options in clang/gcc ?)

Originally posted by @smarter in #6291 (comment)

[som-snytt]

-language:_ means don't hassle me with so-called options.

For -source options, it could mean take the latest available source level.

Otherwise, it could mean "enable ALL the things!" if those settings are not in conflict.

With -Wconf, one could re-imagine language features as aliases for -Wconf. There is a sense in which -Wconf is more expensive than the language feature look-up, so there could be a cost. Maybe that's an implementation detail.

Edit: it's mentioned on the linked ticket that language options can mean semantic differences that aren't reducible to errors. Even if they were, it's not obvious that reporting has enough context to say, "They used feature L, emit a warning if language.L is not enabled." Or, maybe there is a clever way to emit, "Did you remember to enable language.L?" and then that is "suppressed" if it is enabled. Like "Did you remember to sign the CLA?" and other annoyances.