Remove LinkedHashSet

[Tomboyo]

We use LinkedHashSet throughout the AST to ensure set iteration order is consistent between runs. This is a relic of older test strategies that are on the way out and which relied upon consistent encounter order to pass.

In cases where the name of an anonymous type is dependent upon its position in a list, the ordinal comes from the underlying OAS list and is therefore consistent between runs regardless of Set implementation used thereafter.

Remove LinkedHashSet in favor of Set, which is easier to work with.