GSB: Record the original type for self-derived conformances

[slavapestov]

RequirementSources with Superclass and Concrete kind would
reference a protocol conformance. However in the case where
the concrete type was an existential conforming to itself,
the SelfProtocolConformance does not store the original
type. Since self-conforming existentials don't have any
nested types, we don't really need to store this conformance
at all.

Instead of storing a protocol conformance in the case where
the original type is an existential, just the original type
itself. This allows us to recover the requirement from the
RequirementSource, which is important for the new
implementation of computing redundant requirements.

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

Commit message should read “self-conforming existentials” not “self-derived conformances”. I’m too lazy to change it now