swift234

SIL: Provide SILType APIs to track the representation of existential types.

Give SILType 'getPreferredExistentialRepresentation' and 'canUseExistentialRepresentation' methods, which track what types of container particular existential types may use--fixed-sized, class-constrained, metatype, or box. Allow for existentials to use a specialized representation for certain known concrete types by allowing these methods to take a concrete type. NFC yet, except to replace some ad-hoc verifier conditions with canUseExistentialRepresentation checks where appropriate.

Swift SVN r26062