CFE: implement changes to TOP_MERGE

[leafpetersen]

This pull request specifies a change in the definition of the NNBD_TOP_MERGE algorithm. This is ready for implementation. Tests for this change are landing here:

• TOP_MERGE member override tests..
• Test TOP_MERGE on super-interfaces..

The following tests are currently failing and should pass when the above change is implemented.

language/nnbd/top_merge/top_merge_interfaces_direct_error_test MissingCompileTimeError (expected Pass)
language/nnbd/top_merge/top_merge_interfaces_indirect_error_test MissingCompileTimeError (expected Pass)
language/nnbd/top_merge/top_merge_interfaces_mixin_error_test MissingCompileTimeError (expected Pass)
language/nnbd/top_merge/top_merge_members_direct_error_test MissingCompileTimeError (expected Pass)
language/nnbd/top_merge/top_merge_members_indirect_error_test MissingCompileTimeError (expected Pass)
language/nnbd/top_merge/top_merge_members_mixin_error_test MissingCompileTimeError (expected Pass)

cc @johnniwinther @stefantsov

[johnniwinther]

The change have been implemented but language/nnbd/top_merge/top_merge_members_mixin_error_test is still failing but for different reasons.