ModuleInterface: Print imports shadowed by a cross-import overlay

In some cases this import may be superfluous as it's also
`@_exported` imported by the overlay. However, when the overlay
is hidden and the import not printed, the import can go entierly
missing.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -240,7 +240,8 @@ static void printImports(raw_ostream &out,
   // it's not obvious what higher-level optimization would be factored out here.
   ModuleDecl::ImportFilter allImportFilter = {
       ModuleDecl::ImportFilterKind::Exported,
-      ModuleDecl::ImportFilterKind::Default};
+      ModuleDecl::ImportFilterKind::Default,
+      ModuleDecl::ImportFilterKind::ShadowedByCrossImportOverlay};
 
   // With -experimental-spi-imports:
   // When printing the private or package swiftinterface file, print implementation-only