[AutoDiff] Add @differentiable attribute SILGen assertion. (#27650)

Previously, `@differentiable` attribute SILGen lowering was skipped if the
attribute did not have resolved parameter indices. This led to unexpected
and difficult-to-debug bugs like TF-888.

Re-add the assertion for robustness. This is possible after
#27613, which fixed the underlying issue
with parameter indices resolution.

Author: dan-zheng

lib/SIL/SILFunctionBuilder.cpp

@@ -86,10 +86,7 @@ void SILFunctionBuilder::addFunctionAttributes(SILFunction *F,
     for (auto *A : Attrs.getAttributes<DifferentiableAttr>()) {
       // Get lowered argument indices.
       auto *paramIndices = A->getParameterIndices();
-      // NOTE: If `A->getParameterIndices()` is `nullptr`, continue. This is a
-      // necessary hack regarding deserialization.
-      if (!paramIndices)
-        continue;
+      assert(paramIndices && "Parameter indices should have been resolved");
       auto *loweredParamIndices = autodiff::getLoweredParameterIndices(
           paramIndices, decl->getInterfaceType()->castTo<AnyFunctionType>());
       SILAutoDiffIndices indices(/*source*/ 0, loweredParamIndices);