[NFC] Clarify semantics of getImportedModules.

The lack of clarity manifested as unexpected behavior when using
getImportedModules to create the module import graph. The new behavior
makes SPI-ness and Shadowing-ness behave similarly in terms of
filtering. We also check if a filter is well-formed to avoid
accidental empty import lists.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -116,7 +116,8 @@ static void printImports(raw_ostream &out,
 
     SmallVector<ModuleDecl::ImportedModule, 4> ioiImport;
     M->getImportedModules(ioiImport,
-                          ModuleDecl::ImportFilterKind::ImplementationOnly);
+                          {ModuleDecl::ImportFilterKind::ImplementationOnly,
+                           ModuleDecl::ImportFilterKind::SPIAccessControl});
     ioiImportSet.insert(ioiImport.begin(), ioiImport.end());
   }