[AutoDiff] Add 'SILDifferentiableFunctionType'.

[rxwei]

WIP

SILDifferentiableBundleType is a SIL-only first-class type that represents a differentiable function. Previously, we used a @differentiable attribute on normal functions and made functions with that attribute be lowered to a struct. However, that had a ton of problems (TF-11, TF-17, TF-171) so that we had to turn off LoadableByAddress.

The right solution, which @marcrasi initially suggested, is to store the autodiff-associated function types into differentiable SIL function types. More specifically, we want to store the type of all parameters' and results' member types TangentVector and CotangentVector, which are used in differentials and pullbacks, respectively.

This PR is doing just that. A SILDifferentiableFunctionType contains the following information:

• The max differentiation order, e.g. 2.
• The differentiability representation kind
  • Linear: Represents a linear map. maxOrder must be -1.
  • Normal: Represents a non-linear differentiable function. maxOrder must be greater than 0.
• The original function type, e.g. @convention(thin) (T) -> U.
• The differential type, e.g. @callee_guaranteed (T.TangentVector) -> U.TangentVector.
• The pullback type, e.g. @callee_guaranteed (U.CotangentVector) -> T.CotangentVector.

Putting things together, a Swift order-1 differentiable function @differentiable (T) -> U in SIL looks like:

$@differentiable(1) {@convention(thin) (T) -> U, differential: @callee_guaranteed (T.TangentVector) -> U.TangentVector, pullback: @callee_guaranteed (U.CotangentVector) -> T.CotangentVector}

A linear function (e.g. a differential or a pullback) looks like:

$@differentiable(linear) {@convention(thin) (T.TangentVector) -> U.CotangentVector, transpose: @callee_guaranteed (U.CotangentVector) -> T.CotangentVector}