When checking protocol conformances with `@differentiable` requirements, the type checker is supposed to accept omissions of `@differentiable` attributes when there exsits an attribute that covers a superset of the differentiation configuration. This was accidentally regressed in apple/swift#33776 which made the following test case fail to compile. This is fixed by adjusting the witness matching conditions.
```swift
// rdar://70348904 reproducer:
public protocol P: Differentiable {
@differentiable(wrt: self)
@differentiable(wrt: (self, x))
func foo(_ x: Float) -> Float
}
public struct S: P {}
extension S {
// This had worked until apple/swift#33776.
@differentiable(wrt: (self, x))
public func foo(_ x: Float) -> Float { x }
}
```
Also fix some suboptimal diagnostics where more information could be shown.
Resolves rdar://70348904.
During protocol witness matching for a protocol requirement with
`@differentiable` attributes, implicit `@differentiable` attributes may be
created for the witness under specific conditions (when the witness has a
`@differentiable` attribute with superset differentiability parameters, or
when the witness has less-than-public visibility).
Do not generate implicit `@differentiable` attributes for protocol witnesses
when the protocol conformance is declared from a separate file or type context
from the witness.
Resolves SR-13455.
