[AutoDiff] Diagnose non-differentiable original function arguments/results.

[dan-zheng]

Diagnose non-differentiable original function arguments/results in
VJPEmitter::visitApplyInst. Perform the check again if original function
is specialized via partial application.

Resolves TF-687.

Note: this PR depends on #26404 - update and test after it is merged.

---

struct TF_687<T> : Differentiable {
  @noDerivative var indirectDummy: T
  var base: Float

  init(_ base: Float, dummy: T) {
    self.base = base
    self.indirectDummy = dummy
  }
}
let _: @differentiable (Float) -> TF_687<Any> = { x in TF_687<Any>(x, dummy: x) }

Before:

$ swift tf-687.swift
Assertion failed: (paramTan && "Parameter type does not have a tangent space?"), function getAutoDiffAssociatedFunctionType, file /usr/local/src/swift-build/swift/lib/SIL/SILFunctionType.cpp, line 266.
Stack dump:
0.	Program arguments: /Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2019-07-25-a.xctoolchain/usr/bin/swift -frontend -interpret tf-687.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 #668 SILModuleTransform "Differentiation".
0  swift                    0x0000000105154ad5 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x0000000105153b18 llvm::sys::RunSignalHandlers() + 248
2  swift                    0x00000001051550c8 SignalHandler(int) + 264
3  libsystem_platform.dylib 0x00007fff65936b5d _sigtramp + 29
4  libsystem_platform.dylib 0x0000000111e2f938 _sigtramp + 2890894840
5  libsystem_c.dylib        0x00007fff657f06a6 abort + 127
6  libsystem_c.dylib        0x00007fff657b920d basename_r + 0
7  swift                    0x000000010541b1d3 swift::SILFunctionType::getAutoDiffAssociatedFunctionType(swift::AutoDiffIndexSubset*, unsigned int, unsigned int, swift::AutoDiffAssociatedFunctionKind, swift::SILModule&, llvm::function_ref<llvm::Optional<swift::ProtocolConformanceRef> (swift::CanType, swift::Type, swift::ProtocolDecl*)>, swift::CanGenericSignature) (.cold.22) + 35
8  swift                    0x0000000101d42f15 swift::SILFunctionType::getAutoDiffAssociatedFunctionType(swift::AutoDiffIndexSubset*, unsigned int, unsigned int, swift::AutoDiffAssociatedFunctionKind, swift::SILModule&, llvm::function_ref<llvm::Optional<swift::ProtocolConformanceRef> (swift::CanType, swift::Type, swift::ProtocolDecl*)>, swift::CanGenericSignature) + 3477
9  swift                    0x0000000101d6f5a2 swift::AutoDiffFunctionExtractInst::getExtracteeType(swift::SILValue, swift::AutoDiffFunctionExtractInst::Extractee, unsigned int, swift::SILModule&) + 162
10 swift                    0x0000000101d6f64c swift::AutoDiffFunctionExtractInst::AutoDiffFunctionExtractInst(swift::SILModule&, swift::SILDebugLocation, swift::AutoDiffFunctionExtractInst::Extractee, unsigned int, swift::SILValue) + 44
11 swift                    0x0000000101a920a4 (anonymous namespace)::VJPEmitter::visitApplyInst(swift::ApplyInst*) + 7764
12 swift                    0x0000000101a6c449 (anonymous namespace)::ADContext::processDifferentiableAttribute(swift::SILFunction*, swift::SILDifferentiableAttr*, (anonymous namespace)::DifferentiationInvoker) + 10233
13 swift                    0x0000000101abb1ca (anonymous namespace)::ADContext::promoteToDifferentiableFunction(swift::AutoDiffFunctionInst*, swift::SILBuilder&, swift::SILLocation, (anonymous namespace)::DifferentiationInvoker) + 4954
14 swift                    0x0000000101a6f63a (anonymous namespace)::ADContext::processAutoDiffFunctionInst(swift::AutoDiffFunctionInst*) + 378
15 swift                    0x0000000101a69501 (anonymous namespace)::Differentiation::run() + 2913

After:

$ swift tf-687.swift
tf-687.swift:10:49: error: function is not differentiable
let _: @differentiable (Float) -> TF_687<Any> = { x in TF_687<Any>(x, dummy: x) }
                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tf-687.swift:10:78: note: cannot differentiate through a non-differentiable argument; do you want to use 'withoutDerivative(at:)'?
let _: @differentiable (Float) -> TF_687<Any> = { x in TF_687<Any>(x, dummy: x) }
                                                                             ^

[dan-zheng]

Note: this logic should be ported to JVPEmitter::visitApply in #26057.
cc @bartchr808

[bartchr808]

Made a note of this to be a TODO item for myself! 😄

[dan-zheng]

@swift-ci Please test tensorflow