Used assembly analysis does not take into account anonymous types

When moving our tests to target `net8.0` a number of them are failing the used assembly leg with the following style of error message:

```
// error CS0656: Missing compiler required member 'System.Collections.Generic.EqualityComparer`1.Equals'
Diagnostic(ErrorCode.ERR_MissingPredefinedMember).WithArguments("System.Collections.Generic.EqualityComparer`1", "Equals").WithLocation(1, 1),
// error CS0656: Missing compiler required member 'System.Collections.Generic.EqualityComparer`1.GetHashCode'
Diagnostic(ErrorCode.ERR_MissingPredefinedMember).WithArguments("System.Collections.Generic.EqualityComparer`1", "GetHashCode").WithLocation(1, 1),
// error CS0656: Missing compiler required member 'System.Collections.Generic.EqualityComparer`1.get_Default'
Diagnostic(ErrorCode.ERR_MissingPredefinedMember).WithArguments("System.Collections.Generic.EqualityComparer`1", "get_Default").WithLocation(1, 1)
```

This _appears_ to be because the used assembly logic isn't considering the methods on anonymous types in the analysis.

Start by reproducing the net8.0 used-assembly failures described in the issue and trace how used assembly analysis handles anonymous types. Compare the members collected for anonymous types with the missing EqualityComparer members in the diagnostics; done means the affected tests no longer report those missing compiler-required members.

Written by the indexing model from the issue text.