The original bug was a crash-on-invalid with a missing '}', but it
actually exposed a bug with nested protocols (SE-0404) and another
long-time bug.
- Whatever we do, we should skip this for protocols because their 'Self'
parameter is not bound from context.
- getTrailingWhereClause() is not the right proxy for "has a generic
signature different than its parent", in particular it doesn't
round-trip through serialization. Instead, just compare generic
signatures for pointer equality in the early return check.
The second change is source-breaking because it was possible to
write a nested type with a `where` clause and use it contradicting
its requirements across a module boundary.
Fixes rdar://113103854.
