[AutoDiff] NFC: add `SILFunctionType` method assertion. (#29541)
Assert that `SILFunctionType::getWithDifferentiability` is called only
with `DifferentiabilityKind::Normal` or `DifferentiabilityKind::Linear`.
`DifferentiabilityKind::NonDifferentiable` is invalid.
A `isDifferentiable()` assertion already exists in
`SILFunctionType::getDifferentiationParameterIndices`.
No assertion is needed for `SILFunctionType::getWithoutDifferentiability`
because it works for both differentiable and non-differentiable function types.
