[Serialization] Preserve submodule import paths through serialization.

This is a bit of a hack, but Clang submodules are the only case we have to
support.

Part of <rdar://problem/13140302>

Swift SVN r20287

include/swift/Serialization/ModuleFormat.h

@@ -355,7 +355,10 @@ namespace input_block {
   using ImportedModuleLayout = BCRecordLayout<
     IMPORTED_MODULE,
     BCFixed<1>, // exported?
-    BCBlob // module name, optionally followed by a null and then an import path
+    BCFixed<1>, // scoped?
+    BCBlob // module name, with submodule path pieces separated by \0s.
+           // If the 'scoped' flag is set, the final path piece is an access
+           // path within the module.
   >;
 
   using LinkLibraryLayout = BCRecordLayout<