Requirement lowering only expects that it won't see two requirements
of the same kind (except for conformance requirements). So only mark
those as conflicting.
This addresses a crash-on-invalid and improves diagnostics for
move-only generics, because a conflict won't drop the copyability
of a generic parameter and expose a move-only-naive user to
confusing error messages.
Fixes#61031.
Fixes#63997.
Fixes rdar://problem/111991454.
Remove the logic which incorrectly attempted to simulate the
GenericSignatureBuilder's minimization behavior in the presence
of conflicts, since it doesn't matter anymore.
Instead of keeping track of the best one so far, and unifying subsequent rules
against it.
This allows us to record more identities, fixing a regression from an earlier
change where we were unable to eliminate an obviously-redundant rule.
