Fix unrelated_type_equality_checks when comparing enums

[davidmartos96]

Description

Currently the unrelated_type_equality_checks doesn't work when comparing different enums. This can be a gotcha when comparing similar enums in terms of context.

enum EnumOpKind1 { insert, update, delete }
enum EnumOpKind2 { upsert, delete }

void fun() {
  var x = EnumOpKind1.delete;
  if (x == EnumOpKind1.delete) print('delete'); // OK
  if (x == EnumOpKind2.delete) print('delete'); // This should lint
}

Same thing happens when comparing different Mixins. Although this I've noticed when I looked into the source code of the lint.

[coveralls]

Coverage: 96.43% (+0.0004%) from 96.429% when pulling c73c127 on davidmartos96:feat/fix_enum_unrelated_equality into ca2a9fe on dart-lang:main.

[srawlins]

I'd actually prefer we address mixins separately. Mixins can have superconstraints (mixin M on C), and we might want to consider that. I think it is less cut-and-dry than enums. I think two mixins M1 and M2 could be considered related if they each have the same superconstraint... but I could be wrong. I think it is exceedingly rare for an element's static type to be a mixin...

[davidmartos96]

@srawlins Should I leave the "basic" Mixins handling or do you prefer them to be handled in a separate commit?

[srawlins]

I think even the || leftElement is MixinElement code would be better done in a separate PR.

[srawlins]

❤️ Love it. I need to check internal code for new violations, then we can land.

[davidmartos96]

Great, thank you!

[srawlins]

Internal looks clean. Will check flutter at the linter roll to the Dart SDK.