[GSB] Eliminate self-derived nested-type-name-match constraints..
Nested-type-name-match constraints are odd because they are
effectively artifacts of the GenericSignatureBuilder's algorithm,
i.e., they occur when we have two PotentialArchetypes representing the
same type, and each of those PA's has a nested type based on the same
associated type. Because of this, nested-type-name-match constraints
have no useful requirement sources, so the normal means of detecting
self-derived constraints doesn't suffice, and we end up with
self-derived nested-type-name-match constraints eliminating the
constraints they depend on, causing spurious "redundant same-type
constraint" diagnostics and minimized generic signatures that drop
important requirements.
Handle nested-type-name-match constraints by first keeping them out of
the connected-components algorithm used to compute same-type
constraints within an equivalence class. Then, detect self-derived
nested-type-name-match constraints by determining whether any of the
ancestor potential archetypes are in the same equivalence class... and
redundant with the edge that makes the ancestor potential archetypes
equivalent. Remove such self-derived edges, and treat all other
nested-type-name-match edges as derived, providing a minimized
signature.
Fixes SR-5841, SR-5601, and SR-5610
