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.
The GenericSignatureBuilder did not actually allow this, but it's conflict
detection was imperfect so it did not flag the example in the radar;
variations on this code were rejected by the GenericSignatureBuilder.
Since the Requirement Machine finds all conflicts correctly, relax the
logic here to make this example work.
Fixes rdar://problem/91637621.
