[AutoDiff] Fix differentiation for non-wrt inout parameters.

[dan-zheng]

Fix SIL differential function type calculation to handle non-wrt inout
parameters.

Patch SILFunctionType::getDifferentiabilityResultIndices to prevent returning
empty result indices for @differentiable function types with no formal results
where all inout parameters are @noDerivative. TF-1305 tracks a robust fix.

Resolves SR-13305.

Exposes TF-1305: parameter/result differentiability hole for inout parameters.

[dan-zheng]

Quick paste of solution ideas from TF-1305:

• Add new SILParameterDifferentiability kinds, e.g. InoutParameterNotDifferentiableParameter (case 3) and InoutParameterNotDifferentiableResult (case 4).
  • Consider whether this needs to be exposed in AST @differentiable function types.
• Find some way to store parameter/result differentiability in SILFunctionType instead of individual SILParameterInfo/SILResultInfo.

[dan-zheng]

@swift-ci Please test

[dan-zheng]

@rxwei mentioned a nuanced point, which is that there are no clear use cases for inout parameters that are differentiability parameters but not differentiability results (case 4).

That case is indeed not currently expressible, so we can deprioritize support for it: I clarified that in the comment. Supporting that case would involve something like adding a results: clause to @differentiable declaration attribute:

// Pseudo-syntax for supporting case 4:
// `inout` parameter that is a differentiability parameter but not a differentiability result.
@differentiable(wrt: inoutParam, results: return)
func foo(_ inoutParam: inout Float) -> Float {
  return inoutParam
}

The focus of this PR is supporting inout parameters that are differentiability results but that are not differentiability parameters (case 3), which is currently expressible and reasonable to support.

// Case 3: `inout` parameter that is a differentiability result but not a differentiability parameter.
@differentiable(wrt: x)
func foo(_ x: Float, _ inoutParam: inout Float) {
  inoutParam = x * x
}

[dan-zheng]

@swift-ci Please test

[swift-ci]

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[swift-ci]

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