[AutoDiff upstream] Add SIL differentiability witnesses.

[dan-zheng]

SIL differentiability witnesses are a new top-level SIL construct mapping
"original" SIL functions to derivative SIL functions.

SIL differentiability witnesses have the following components:

• "Original" SILFunction.
• SIL linkage.
• Differentiability parameter indices (IndexSubset).
• Differentiability result indices (IndexSubset).
• Derivative GenericSignature representing differentiability generic requirements (optional).
• JVP derivative SILFunction (optional).
• VJP derivative SILFunction (optional).
• "Is serialized?" bit.

sil_differentiability_witness hidden [parameters 0] [results 0] <T where T : Differentiable> @id : $@convention(thin) (T) -> T {
  jvp: @id_jvp : $@convention(thin) (T) -> (T, @owned @callee_guaranteed (T.TangentVector) -> T.TangentVector)
  vjp: @id_vjp : $@convention(thin) (T) -> (T, @owned @callee_guaranteed (T.TangentVector) -> T.TangentVector)
}

This patch adds the SILDifferentiabilityWitness data structure, with
documentation, parsing, and printing.

Resolves TF-911.

---

Todos:

• TF-1136: upstream SILDifferentiabilityWitness serialization.
• TF-1137: upstream SILDifferentiabilityWitness verification.
• TF-1138: upstream SILDifferentiabilityWitness SILGen from
  @differentiable and @derivative attributes.
• TF-20: robust mangling for SILDifferentiabilityWitness names.

[dan-zheng]

Note: printing SIL differentiability witnesses before SIL functions may be slightly controversial.

This decision significantly simplifies SIL parsing: it allows us to avoid adding support for diff. witness forward declarations when parsing differentiability_witness_function instructions in SIL functions.

Other longer table-like SIL constructs are printed after SIL functions so that functions appear at the top. Diff. witnesses aren't as long as other table-like SIL constructs (up to four lines), so they don't take up too much space.

[dan-zheng]

@jckarter: would you like to review this PR, or suggest a reviewer? I'm asking you since you seem like a SIL code owner.

This PR is part of our effort to upstream differentiable programming. It adds a new top-level SIL construct called "differentiability witnesses" and doesn't affect much existing code.

If there's no activity, I'll merge within a few days to unblock progress. Happy to address feedback at any time!

[dan-zheng]

@swift-ci Please smoke test

[dan-zheng]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 9f7ca52

[jckarter]

Looks good to me. If you're just adding data structures to support autodiff that have minimal interaction with the rest of the compiler like this, I would say don't wait for me.

[dan-zheng]

Looks good to me. If you're just adding data structures to support autodiff that have minimal interaction with the rest of the compiler like this, I would say don't wait for me.

Got it, thank you!

If you don't mind, I'll continue to add you as a reviewer on SIL autodiff upstreaming patches, unless you have some other preference.

[dan-zheng]

The macOS Python/python-lint test failure seems unrelated to this patch, so I'll merge now.