Merge #9144

9144: Extend convert_tuple_struct_to_named_struct to also apply to enum tuple variants r=matklad a=joshuawarner32

This is largely copied and adapted from the existing `convert_tuple_struct_to_named_struct` code.

Not sure if maybe some of this code can/should be shared between those two assists - but the differences are significant enough to make it at least seem like a non-trivial refactor.

Co-authored-by: Joshua Warner <joshuawarner32@gmail.com>

Author: bors[bot]