swift234

AST: Add ArchetypeBuilder::mapTypeOutOfContext(), NFC

This is the inverse of mapTypeIntoContext(). There were three
implementations of similar algorithms elsewhere, in Sema,
SIL type lowering, and some code in GenericParamList that is
only used by the SIL parser.

This new function will replace the first two, and the final one
will be refactored away.