swift234

Serialize cross-references to constrained extensions with a generic signature.

Extensions cannot be uniquely cross-referenced, so cross-references to
extensions are serialized with the extended nominal type name and the
module in which the extension resides. This is not sufficient when
cross-referencing the generic type parameters of a constrained
protocol extension, because we don't know whether to get the
archetypes of the nominal type or some extension thereof. Serialize
the canonical generic signature so that we can pick an extension with
the same generic signature; it doesn't matter which we pick, so long
as we're consistent.

Fixes rdar://problem/20680169. Triggering this involves some
interesting interactions between the optimizer and standard library;
the standard library updates in the radar will test this.

Swift SVN r27825