swift234

IRGen: Share TypeInfo among equivalent generic instantiations, take 2.

IRGen type conversion is invariant with respect to archetypes with the same set of constraints, so instead of redundantly generating a TypeInfo object and IR type for Optional<T> for every T everywhere, key TypeInfo objects using an "exemplar type" that we form using a folding set to collapse together archetypes with the same class-ness, superclass constraint, and protocol constraints.

This is a nice memory and IR size optimization, but will be essential for correctness when lowering interface types, because there is no unique context to ground a dependent type, and we need to lower the same generic parameter with the same context requirements to the same type whenever we instantiate it in order for the IR to type-check.

In this revision, we profile the nested archetypes of each recursively, which I neglected to take into account originally in r12112, causing failures when archetypes that differed by associated type constraints were incorrectly collapsed.

Swift SVN r12116