[AutoDiff] Canonicalize SIL type for JVP/VJP methods.

[dan-zheng]

Canonicalize JVP/VJP type calculation for methods so that JVP/VJP methods return
a linear map that take/return self's tangent/cotangent last, instead of first.

This matches the type calculation logic for top-level functions.

Changes:

• Remove method-specific logic from
  SILFunctionType::getAutoDiffAssociatedFunctionType.
  Handle self like a normal differentiation parameter.
• Thunk user-defined JVP/VJP methods, reordering the position of self's
  tangent/cotangent in returned linear maps.
  • Relevant code:
    SILGenModule::getOrCreateAutoDiffAssociatedFunctionReorderingThunk and
    SILGenFunction::getOrCreateAutoDiffLinearMapReorderingThunk.
• Change type computation for JVP/VJP method protocol witnesses.
  • Relevant code: getSILFunctionType in SILFunctionType.cpp and
    SILGenFunction::emitProtocolWitness.

Move some functions to a common location:

• AnyFunctionType::getAutoDiffOriginalFunctionType
• autodiff::getAutoDiffFunctionLinkage

---

This is a step towards refactoring the differentiation transform (iterative autodiff_function worklist redesign), which will unblock control-flow support.

[dan-zheng]

It's possible to avoid introducing these dependencies by moving the "thunk function creation" logic to SILGenThunk.cpp.

This is done for reabstraction thunks:

• Thunk SILFunction creation is done in SILGenThunk.cpp in SILGenModule::getOrCreateReabstractionThunk. SILGenThunk.cpp includes the appropriate headers.
• Thunk body creation is done here in SILGenPoly.cpp.

Consider refactoring at some point.

[dan-zheng]

@swift-ci Please test tensorflow

[marcrasi]

I have not read through all the type calculation and thunking in detail yet, because I have a high-level comment that might simplify some of it.

Can you make the AST type of the associated functions have self last? i.e. make AnyFunctionType::getAutoDiffAssociatedFunctionType return types with self last?

I haven't thought this through carefully, but I think that if you did that:

• This order wouldn't be exposed to the user anywhere, because the user never gets direct access to that AST type. (Except: You might have to make a small ordering adjustment in TypeCheckAttr).
• This would eliminate the reordering logic in SILFunctionType.cpp, because the order would already be correct.
• This would eliminate the reordering logic in SILGenFuncion::emitProtocolWitness, because the AST types that get there would already be in the right order.
• Also, this would be overall safer and less likely to break in the future, because every internal layer would have the same order. Only the very top layer that's exposed to the user would have a different order.

[dan-zheng]

Can you make the AST type of the associated functions have self last? i.e. make AnyFunctionType::getAutoDiffAssociatedFunctionType return types with self last?

Good idea! We discussed in person and I discussed with @rxwei - we agree that computing self-parameter-reordered JVP/VJP types on AST function types instead of SIL function types is safer and simpler.

Done in 063f4fd, removing 100+ lines from SILGen and adding <10 lines to AST/Sema.

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

@swift-ci Please test tensorflow