Track whether a class is "foreign" in the AST.

This basically just means "it's a CF class" for now,
but you could imagine applying this to all sorts of
class-like types from peer runtimes that we can't
support all possible language features for.

There are quite a few language features that require
fairly deep object-model integration to implement,
like subclassing and adding polymorphic methods.
Some of those features, like final classes, are useful
to generally support as attributes, but most of
them aren't.  At least in the short term, it makes
sense to have a big hammer we can hit things with.

Swift SVN r17428

include/swift/Serialization/ModuleFormat.h

@@ -629,6 +629,7 @@ namespace decls_block {
     BCFixed<1>,        // explicitly objc?
     BCFixed<1>,        // requires stored property initial values (attribute)
     BCFixed<1>,        // requires stored property initial values
+    BCFixed<1>,        // foreign
     TypeIDField,       // superclass
     BCArray<DeclIDField> // protocols
     // Trailed by the generic parameters (if any), the decl context record, and