[AutoDiff] NFC: garden tests. (swiftlang#30598)

* Move muliti-functionality SIL tests to test/AutoDiff/SIL.

* Add `with_derivative` clause to `differentiable_function` instructions.

Otherwise, IRGen for test/AutoDiff/SIL/differentiable_function_inst.sil fails on
tensorflow branch because the differentiation transform cannot differentiate
external functions.

Author: dan-zheng

test/AutoDiff/differentiability_witness_function_inst.sil renamed to test/AutoDiff/SIL/differentiability_witness_function_inst.sil

@@ -57,20 +57,20 @@ sil @examplemethod : $@convention(method) (Float, Float, Float) -> Float
 sil @test_roundtrip_parse : $@convention(thin) () -> () {
 bb0:
   %0 = function_ref @examplefunc : $@convention(thin) (Float, Float, Float) -> Float
-  %1 = differentiable_function [parameters 0 1 2] %0 : $@convention(thin) (Float, Float, Float) -> Float
+  %1 = differentiable_function [parameters 0 1 2] %0 : $@convention(thin) (Float, Float, Float) -> Float with_derivative {undef : $@convention(thin) (Float, Float, Float) -> (Float, @owned @callee_guaranteed (Float, Float, Float) -> Float), undef : $@convention(thin) (Float, Float, Float) -> (Float, @owned @callee_guaranteed (Float) -> (Float, Float, Float))}
 
   // CHECK-SIL: %2 = differentiable_function_extract [vjp] %1 : $@differentiable @convention(thin) (Float, Float, Float) -> Float
   %2 = differentiable_function_extract [vjp] %1 : $@differentiable @convention(thin) (Float, Float, Float) -> Float
-  %3 = differentiable_function [parameters 0] %0 : $@convention(thin) (Float, Float, Float) -> Float
+  %3 = differentiable_function [parameters 0] %0 : $@convention(thin) (Float, Float, Float) -> Float with_derivative {undef : $@convention(thin) (Float, Float, Float) -> (Float, @owned @callee_guaranteed (Float) -> Float), undef : $@convention(thin) (Float, Float, Float) -> (Float, @owned @callee_guaranteed (Float) -> Float)}
 
   // CHECK-SIL: %4 = differentiable_function_extract [vjp] %3 : $@differentiable @convention(thin) (Float, @noDerivative Float, @noDerivative Float) -> Float
   %4 = differentiable_function_extract [vjp] %3 : $@differentiable @convention(thin) (Float, @noDerivative Float, @noDerivative Float) -> Float
   %5 = function_ref @examplemethod : $@convention(method) (Float, Float, Float) -> Float
-  %6 = differentiable_function [parameters 0 1 2] %5 : $@convention(method) (Float, Float, Float) -> Float
+  %6 = differentiable_function [parameters 0 1 2] %5 : $@convention(method) (Float, Float, Float) -> Float with_derivative {undef : $@convention(method) (Float, Float, Float) -> (Float, @owned @callee_guaranteed (Float, Float, Float) -> Float), undef : $@convention(method) (Float, Float, Float) -> (Float, @owned @callee_guaranteed (Float) -> (Float, Float, Float))}
 
   // CHECK-SIL: %7 = differentiable_function_extract [vjp] %6 : $@differentiable @convention(method) (Float, Float, Float) -> Float
   %7 = differentiable_function_extract [vjp] %6 : $@differentiable @convention(method) (Float, Float, Float) -> Float
-  %8 = differentiable_function [parameters 0] %5 : $@convention(method) (Float, Float, Float) -> Float
+  %8 = differentiable_function [parameters 0] %5 : $@convention(method) (Float, Float, Float) -> Float with_derivative {undef : $@convention(method) (Float, Float, Float) -> (Float, @owned @callee_guaranteed (Float) -> Float), undef : $@convention(method) (Float, Float, Float) -> (Float, @owned @callee_guaranteed (Float) -> Float)}
 
   // CHECK-SIL: %9 = differentiable_function_extract [vjp] %8 : $@differentiable @convention(method) (Float, @noDerivative Float, @noDerivative Float) -> Float
   %9 = differentiable_function_extract [vjp] %8 : $@differentiable @convention(method) (Float, @noDerivative Float, @noDerivative Float) -> Float