[AutoDiff] Differentiate partially-applied reabstraction thunks.
Support differentiation of partially-applied reabstraction thunks:
```
%generic_fn = function_ref @generic : $<τ_0_0> (@in_guaranteed τ_0_0) -> @out τ_0_0
%specialized_fn = partial_apply %generic_fn<Float>()
%reabs_thunk = function_ref @reabs_thunk : $(Float, @guaranteed @callee_guaranteed (@in_guaranteed Float) -> @out Float) -> Float
%thunked_fn = partial_apply %reabs_thunk(%specialized_fn)
%diff_fn = differentiable_function %thunked_fn
```
The enables differentiation of direct references to generic functions.
- SIL: add special reabstraction thunk case to
`SILFunctionType::getAutoDiffDerivativeFunctionType`.
- Reabstraction thunks have a function as the last argument.
- Reabstraction thunk JVPs/VJPs have a `@differentiable` function as the last
argument.
- Differentiation transform: handle partially-applied reabstraction thunks in
`reapplyFunctionConversion`.
Resolves TF-201.
