always lookup pure enum symbols if expected type is enum

[metagn]

fixes #23689

Normally pure enum symbols only "exist" in lookup if nothing else with the same name is in scope. But if an expression is expected to be an enum type, we know that ambiguity can be resolved between different symbols based on their type, so we can include the normally inaccessible pure enum fields in the ambiguity resolution in the case that the expected enum type is actually a pure enum. This handles the use case in the issue of the type inference for enums reverted in #23588.

I know pure enums are supposed to be on their way out so this might seem excessive, but the pure pragma can't be removed in the code in the issue due to a redefinition error, they have to be separated into different modules. Normal enums can still resolve the ambiguity here though. I always think about making a list of all the remaining use cases for pure enums and I always forget.

Will close #23694 if CI passes

[github-actions]

Thanks for your hard work on this PR!
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