in GetChildCompilerTypeAtIndex, switch fallback() to impl()

Now that `TypeSystemSwiftTypeRef::GetNumChildren` is implemented, if the
runtime knows of more children than the AST contains, then the runtime should
be used (`impl`), not the AST (`fallback`).

Author: kastiglione

lldb/source/Plugins/TypeSystem/Swift/TypeSystemSwiftTypeRef.cpp

@@ -2345,7 +2345,7 @@ CompilerType TypeSystemSwiftTypeRef::GetChildCompilerTypeAtIndex(
   if (m_swift_ast_context->GetNumChildren(ReconstructType(type),
                                           omit_empty_base_classes, exe_ctx) <
       runtime->GetNumChildren({this, type}, valobj).getValueOr(0))
-    return fallback();
+    return impl();
 
 #ifndef NDEBUG
   std::string ast_child_name;