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`.
