swift234

Move ctor archetype forwarding from TypeChecker to SILGen.

Creating forwarding substitutions is straightforward enough that we don't need to fudge them in the AST, so remove the forwarding substitutions from ConstructorDecl and construct them instead on the fly in SILGen. We're going to need to be able to forward archetypes in other places in SILGen, such as when emitting closures or curried thunks in generic contexts. No functionality change.

Swift SVN r5418