swift

23342a5a - [AutoDiff] Serialize derivative function configurations per module.

[AutoDiff] Serialize derivative function configurations per module. `@differentiable` and `@derivative` attributes register derivatives for `AbstractFunctionDecl`s for a particular "derivative function configuration": parameter indices and dervative generic signature. To find `@derivative` functions registered in other Swift modules, derivative function configurations must be serialized per module. When configurations for a `AbstractFunctionDecl` are requested, all configurations from imported modules are deserialized. This module serialization technique has precedent: it is used for protocol conformances (e.g. extension declarations for a nominal type) and Obj-C members for a class type. Add `AbstractFunctionDecl::getDerivativeFunctionConfigurations` entry point for accessing derivative function configurations. Use `AbstractFunctionDecl::getDerivativeFunctionConfigurations` to implement `findMinimalDerivativeConfiguration` for canonical derivative function configuration lookup, replacing `getMinimalASTDifferentiableAttr`. Unblocks TF-815: lowering `@derivative` attributes directly to SIL differentiability witnesses without generating implicit `@differentiable` attributes.