Optimize checking involving large discriminated union types (#42556)

* No array literal subtype reduction when contextual type is present

* Accept new baselines

* Fast path in relations and filtering of pure discriminated union types

* Create maps for mixed unions, but not for small or primitive only unions

* Create many-to-many mapping with certain limits, also use in CFA

* Use constituent maps in CFA for switch statements, cleanup, add comments

* Revert change to apparent contextual type / better criteria for map eligibility

* Deduplicate array literal element types

* Accept new baselines

* Filter in false case only when discriminant property has unit type

* Only subtype reduce unions with less than 100 distinct types

* Accept new baselines

* Caching and quick discriminant checks in subtype reduction

* Accept new baselines

* Remove deduplication logic now that subtype reduction was optimized

Author: ahejlsberg