swift234

Turn SerializedModule into SerializedASTFile.

Part of the FileUnit restructuring. A serialized module is now represented as
a TranslationUnit containing a single SerializedASTFile.

As part of this change, the FileUnit interface has been made virtual, rather
than switching on the Kind in every accessor. We think the operations
performed on files are sufficiently high-level that this shouldn't affect us.

A nice side effect of all this is that we now properly model the visibility
of modules imported into source files. Previously, we would always consider
the top-level imports of all files within a target, whether re-exported or
not.

We may still end up wanting to distinguish properties of a complete Swift
module file from a partial AST file, but we can do that within
SerializedModuleLoader.

Swift SVN r10832