Get rid of make_simplified_union() except in error messages

[ilevkivskyi]

It used to cause various issues and is still causing known bugs (like false negatives in per-file strict optional when map_instance_to_supertype() is involved). Ideally we should replace all calls to it (except in error message formatting code) with UnionType.make_union(). Union constructor already flattens nested unions, we can also make it remove exact duplicates minimize perf impact.

[Akuli]

Ideally there would be a test case to ensure #9169 won't happen again once the refactoring is done

[97littleleaf11]

I've noticed that make_simplified_union has been modified several times since this issue opened:

• Speed up make_simplified_union for Literal[string]s (issue #9169) #9192 adds a fast path for union of literal strings and later typeops: extend make_simplified_union fast path to enums #9394 extends the fast path to enums
• Recombine complete union of enum literals into original type (#9063) #9097 adds a helper function for combining union of enum literals
• Fix make_simplified_union for instances with last_known_value #10373 fixes instances with last_known_value by adding a helper func is_redundant_literal_instance

We probably need to reconsider the necessity and ways to refactor make_union and make_simplified_union