swift234

Sketch out the interface to an iterative, lazy type checker.

This is all effectively NFC, but lays out the shape of the iterative
type checker: requests are packaged up in TypeCheckRequest, we can
check whether the request has been satisfied already (isSatisfied),
enumerate its dependencies (enumerateDependenciesOf) in terms of other
TypeCheckRequests, and satisfy a request (satisfy).

Lazily-computed semantic information is captured directly in the
AST, but has been set aside in its own structure to allow us to
experiment with moving it into a lookaside table.

The only request that exists now is to type-check the superclass of
the given class. It currently performs unhealthy recursion into the
existing type checker. As we detangle dependencies, this recursion
between the IterativeTypeChecker and the TypeChecker can go away.

Swift SVN r32558