use irgen::ASTVisitor, which is a subclass of the AST version
that always looks through some expressions and asserts that
others (the unchecked/error ones) can't happen. The main
obstacle to that is access control, because it's quite nice to
have the visitor be a non-local class for nesting purposes,
but that makes it impossible to use private methods anymore.
It's nice to give all the type-specific emitters private
interfaces anyway, though; that ends up being most of the
patch.
I futzed with GenClosure.cpp while I was in there, but
separating it would have been obnoxious.
Swift SVN r1267
