[AutoDiff] Constrain wrt parameters to conform to Differentiable.

[dan-zheng]

Constrain all wrt parameters to conform to Differentiable when computing
AD associated function generic signatures.

This fixes crashes when differentiating generic original functions that
do not constrain parameters to be Differentiable, e.g. an unconstrained
identity function.

Gardening included:

• Remove unused isSerialized flag from
  SILGenModule::getOrCreateAutoDiffAssociatedFunctionThunk.
• Rename whereClauseGenericSignature in SIL to
  associatedFunctionGenericSignature.
  • The generic signature does not necessarily come from the where
    clause of a [differentiable] attribute.

Resolves TF-691 and TF-697.

---

TF-691 reproducer:

// tf-691.swift
func id<T>(_ x: T) -> T { x }
print(gradient(at: 0, in: { x in id(x) }))

Before:

$ swift tf-691.swift
Stack dump:
0.	Program arguments: /Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2019-07-25-a.xctoolchain/usr/bin/swift -frontend -interpret tf-691.swift -enable-objc-interop -sdk /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -color-diagnostics -module-name main
1.	Swift version 5.1-dev (LLVM 200186e28b, Swift 3416770d73)
2.	While running pass #28 SILModuleTransform "Differentiation".
0  swift                    0x000000010ae97ad5 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x000000010ae96b18 llvm::sys::RunSignalHandlers() + 248
2  swift                    0x000000010ae980c8 SignalHandler(int) + 264
3  libsystem_platform.dylib 0x00007fff65936b5d _sigtramp + 29
4  libsystem_platform.dylib 0x00007fc6038c0430 _sigtramp + 2650314992
5  swift                    0x0000000107a8a803 (anonymous namespace)::SILTypeSubstituter::substSILFunctionType(swift::CanTypeWrapper<swift::SILFunctionType>) + 243
6  swift                    0x0000000107a8a6d7 swift::SILFunctionType::substGenericArgs(swift::SILModule&, swift::SubstitutionMap) + 231
7  swift                    0x0000000107afc59d swift::SILType::substGenericArgs(swift::SILModule&, swift::SubstitutionMap) const + 77
8  swift                    0x0000000107ab1dc6 swift::PartialApplyInst::create(swift::SILDebugLocation, swift::SILValue, llvm::ArrayRef<swift::SILValue>, swift::SubstitutionMap, swift::ParameterConvention, swift::SILFunction&, swift::SILOpenedArchetypesState&, swift::GenericSpecializationInformation const*, swift::PartialApplyInst::OnStackKind) + 70
9  swift                    0x00000001077cfc1d swift::SILInstructionVisitor<(anonymous namespace)::VJPEmitter, void>::visit(swift::SILInstruction*) + 50573
10 swift                    0x00000001077af6b8 (anonymous namespace)::ADContext::processDifferentiableAttribute(swift::SILFunction*, swift::SILDifferentiableAttr*, (anonymous namespace)::DifferentiationInvoker) + 10856
11 swift                    0x00000001077fe1ca (anonymous namespace)::ADContext::promoteToDifferentiableFunction(swift::AutoDiffFunctionInst*, swift::SILBuilder&, swift::SILLocation, (anonymous namespace)::DifferentiationInvoker) + 4954
12 swift                    0x00000001077b263a (anonymous namespace)::ADContext::processAutoDiffFunctionInst(swift::AutoDiffFunctionInst*) + 378
13 swift                    0x00000001077ac501 (anonymous namespace)::Differentiation::run() + 2913

After:

$ swift tf-691.swift
1.0

---

TF-697 reproducer:

// tf-697.swift
// TF-697: Test generic requirements of generated AD associated function.
protocol Module: Differentiable where AllDifferentiableVariables == TangentVector {
  associatedtype Input
  associatedtype Output: Differentiable

  @differentiable(wrt: self)
  func callAsFunction(_ input: Input) -> Output
}
protocol Layer: Module where Input: Differentiable {
  @differentiable
  func callLayer(_ input: Input) -> Output
}
struct Sequential<Layer1: Module, Layer2: Layer>: Module
  where Layer1.Output == Layer2.Input {
  var layer1: Layer1
  var layer2: Layer2

  @differentiable(wrt: self)
  func callAsFunction(_ input: Layer1.Input) -> Layer2.Output {
      layer2(layer1(input))
  }
}
extension Sequential: Layer where Layer1: Layer {
  @differentiable
  func callAsFunction(_ input: Layer1.Input) -> Layer2.Output {
      layer2(layer1(input))
  }
}

Before:

$ swift tf-697.swift
SIL verification failed: JVP type does not match expected JVP type
  $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Module, τ_0_1 : Layer, τ_0_0.AllDifferentiableVariables == τ_0_0.TangentVector.AllDifferentiableVariables, τ_0_0.Input : Differentiable, τ_0_0.Output == τ_0_1.Input, τ_0_0.AllDifferentiableVariables.VectorSpaceScalar == τ_0_1.AllDifferentiableVariables.VectorSpaceScalar> (@in_guaranteed τ_0_0.Input, @in_guaranteed Sequential<τ_0_0, τ_0_1>) -> (@out τ_0_1.Output, @owned @callee_guaranteed (@in_guaranteed Sequential<τ_0_0, τ_0_1>.AllDifferentiableVariables) -> @out τ_0_1.Output.TangentVector)
  $@convention(method) <τ_0_0, τ_0_1 where τ_0_0 : Module, τ_0_1 : Layer, τ_0_0.AllDifferentiableVariables == τ_0_0.TangentVector.AllDifferentiableVariables, τ_0_0.Output == τ_0_1.Input, τ_0_0.AllDifferentiableVariables.VectorSpaceScalar == τ_0_1.AllDifferentiableVariables.VectorSpaceScalar> (@in_guaranteed τ_0_0.Input, @in_guaranteed Sequential<τ_0_0, τ_0_1>) -> (@out τ_0_1.Output, @owned @callee_guaranteed (@in_guaranteed Sequential<τ_0_0, τ_0_1>.AllDifferentiableVariables) -> @out τ_0_1.Output.TangentVector)
...

After:

$ swift tf-697.swift
# No error.

[dan-zheng]

I thought about unifying getAssociatedFunctionGenericSignature defined here in lib/SIL/SILFunctionType.cpp (1) with the one defined in lib/SILOptimizer/Differentiation.cpp (2), but the two are not quite compatible.

(1) takes a pre-existing associated function generic signature, but (2) builds the associated function generic signature given the original function's generic signature and [differentiable] attribute requirements.

The part that can be shared is the "constrain all wrt parameters to conform to Differentiable" logic - I don't sharing that short code is worthwhile.

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

Merging to unblock swift-DEVELOPMENT-SNAPSHOT-2019-07-28-a merge (#26398), happy to address feedback later.