[NFC] Convert DWARF Module import graph query into assertion

DWARF-imported modules should have no external dependencies on other modules. This call will just wind up returning the same result every time. Let's use it to assert that this is always the case going forward.

Author: CodaFi

lib/ClangImporter/DWARFImporter.cpp

@@ -118,7 +118,9 @@ ModuleDecl *ClangImporter::Implementation::loadModuleDWARF(
   decl->addFile(*wrapperUnit);
 
   // Force load overlay modules for all imported modules.
-  (void) namelookup::getAllImports(decl);
+  assert(namelookup::getAllImports(decl).size() == 1 &&
+         namelookup::getAllImports(decl).front().importedModule == decl &&
+         "DWARF module depends on additional modules?");
 
   // Register the module with the ASTContext so it is available for lookups.
   if (!SwiftContext.getLoadedModule(name))