This change fixes a bug in which two interface types were being
    incorrectly commoned (considered identical) in the initial stages of
    writing out types to export data. The indexer does a walk to collect
    candidates for export, inserting types into a table to eliminate
    duplicates; as part of this process a local interface type T1 was
    being commoned with a different interface type T2. This caused a cycle
    in the exported type graph due to the way embedded interfaces are
    handled.
    
    The fix was to add a new flag to the Type::is_identical utility
    routine to request that interface type comparison be done by examining
    the original parse methods, as opposed to the expanded method set,
    then use the new flag when creating the hash map for the exporter.
    
    Fixes golang/go#30659.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/166638

From-SVN: r269634