Fix AutoDiff/differentiable_attr_type_checking.swift.

Previously, `ProtocolConformanceRef::getTypeWitnessByName` returned a null type.
Now, it returns a dependent member type with an error base type:

```
(dependent_member_type assoc_type=Swift.(file).Differentiable.TangentVector
  (base=error_type
    (original_type=bound_generic_struct_type decl=main.(file).TF_521@tf-521.swift:1:8
      (primary_archetype_type address=0x7fe08d860660
                              conforms_to=Swift.(file).Differentiable
                              conforms_to=Swift.(file).FloatingPoint name=T
        (nested_type=Exponent <<unresolved>>)
        (nested_type=IntegerLiteralType <<unresolved>>)
        (nested_type=Magnitude <<unresolved>>)
        (nested_type=Stride <<unresolved>>)
        (nested_type=TangentVector =T)))))
```

Explicitly check whether types have error in `conformsToDifferentiable`.

Author: dan-zheng

lib/Sema/TypeCheckAttr.cpp

@@ -2670,7 +2670,7 @@ static bool conformsToDifferentiable(Type type, DeclContext *DC) {
   // Try to get the `TangentVector` type witness, in case the conformance has
   // not been fully checked and the type witness cannot be resolved.
   Type tanType = conf->getTypeWitnessByName(type, ctx.Id_TangentVector);
-  return !tanType.isNull();
+  return !tanType.isNull() && !tanType->hasError();
 };
 
 // SWIFT_ENABLE_TENSORFLOW