Referencing subclass from superclass initializer might lead to class loading deadlock here:

• https://github.com/antlr/antlr4/blob/dev/runtime/Java/src/org/antlr/v4/runtime/atn/SemanticContext.java#L35
• https://github.com/antlr/antlr4/blob/dev/runtime/Java/src/org/antlr/v4/runtime/atn/PredictionContext.java#L28
• https://github.com/antlr/antlr4/blob/dev/runtime/Java/src/org/antlr/v4/runtime/RuleContext.java#L69

And spread changes to other runtimes.