[Sema] Diagnose redundant conditional conformances more specifically.

[huonw]

We don't allow things like

extension Type: P where Param: P {}
extension Type: P where Param: Q {}

or

extension Type: P where Param == Int {}
extension Type: P where Param == Float {}

or

extension Type: P where Param: P {}
extension Type: P where Param == SomethingThatIsntP {}

and the default error message "redundant conformance" message doesn't convey
this very well.

Fixes rdar://problem/36262409.

[huonw]

@swift-ci please smoke test