Consider moving some whole-module diagnostics to FirLanguageVersionSettingsChecker

[ZacSweers]

Would need setting up a CLI test: https://github.com/JetBrains/kotlin/blob/4acabd59448ed256aa0a771e73b264a903e72660/compiler/tests-integration/testFixtures/org/jetbrains/kotlin/cli/AbstractCliTest.java#L45