Update expected text for differentiable warnings

Author: texasmichelle

test/AutoDiff/downstream/differentiable_requirement_cross_module.swift

@@ -26,7 +26,7 @@ struct InternalConforming : DifferentiableRequirement {
 
 // expected-error @+1 {{type 'PublicConforming' does not conform to protocol 'DifferentiableRequirement'}}
 public struct PublicConforming : DifferentiableRequirement {
-  // expected-note @+1 {{candidate is missing attribute '@differentiable(wrt: float)'}}
+  // expected-note @+1 {{candidate is missing explicit '@differentiable(wrt: float)' attribute to satisfy requirement 'foo(float:empty:)' (in protocol 'DifferentiableRequirement')}}
   public func foo(float: Float, empty: Empty) -> Float {
     return float
   }

test/AutoDiff/downstream/protocol_requirement_autodiff_diags.swift

@@ -13,6 +13,6 @@ public struct AttemptsToSatisfyRequirement: HasRequirement {
   // This does not satisfy the requirement because the differentiable attribute is more
   // constrained than the requirement's differentiable attribute.
   @differentiable(where T: P)
-  // expected-note @+1 {{candidate is missing attribute '@differentiable(wrt: (x, y))'}}
+  // expected-note @+1 {{candidate is missing explicit '@differentiable(wrt: (x, y))' attribute to satisfy requirement 'requirement' (in protocol 'HasRequirement')}}
   public func requirement<T: Differentiable>(_ x: T, _ y: T) -> T { x }
 }