[AutoDiff] Support `inout` argument differentiation. (#30013)
Cherry-pick https://github.com/apple/swift/pull/29959:
AST and SIL typing rules for `inout` parameter differentiation.
---
Add reverse-mode differentiation support for `apply` with `inout` arguments.
Notable pullback generation changes:
- If the pullback seed argument is `inout`, assign it (rather than a copy)
directly as the adjoint buffer of the original result. This is important so
the value is updated in-place.
- In `visitApplyInst`: skip adjoint accumulation for `inout` arguments.
Adjoint accumulation for `inout` arguments occurs when callee pullbacks are
applied, so no extra accumulation is necessary.
Add derivatives for functions with `inout` parameters in the stdlib for testing:
- `FloatingPoint` operations: `+=`, `-=`, `*=`, `/=`
- `Array.append`
Resolves TF-1165.
Todos:
- Add more tests, e.g. SILGen tests for `inout` derivative typing rules.
- Evaluate performance of `inout` derivatives vs functional derivatives + mutation.
- TF-1166: enable `@differentiable` attribute on `set` accessors.
- TF-1173: add forward-mode differentiation support for `apply` with `inout`
parameters.
Exposes TF-1175: incorrect activity for class arguments.
Exposes TF-1176: incorrect activity for class `modify` accessors.
Add negative tests.
