Add clarifying comments.

Author: dan-zheng

lib/SILGen/SILGenPoly.cpp

@@ -3695,6 +3695,8 @@ SILGenModule::getOrCreateAutoDiffDerivativeFunctionThunk(
   SILGenFunctionBuilder fb(*this);
   auto linkage = autodiff::getAutoDiffDerivativeFunctionLinkage(
       original->getLinkage(), /*isDerivativeFnExported*/ true);
+  // This thunk is publicly exposed and cannot be transparent.
+  // TODO(TF-925): Mark the thunks as "always inline" for optimization.
   auto *thunk = fb.getOrCreateFunction(
       loc, name, linkage, origDerivativeFnType, IsBare, IsNotTransparent,
       derivativeFn->isSerialized(), derivativeFn->isDynamicallyReplaceable(),

lib/SILGen/SILGenThunk.cpp

@@ -86,6 +86,8 @@ SILGenModule::getOrCreateAutoDiffThunk(SILDeclRef derivativeFnDeclRef,
   auto linkage = autodiff::getAutoDiffDerivativeFunctionLinkage(
       originalLinkage, /*isDerivativeFnExported*/ true);
   auto name = derivativeFnDeclRef.mangle();
+  // This thunk is publicly exposed and cannot be transparent.
+  // TODO(TF-925): Mark the thunks as "always inline" for optimization.
   auto *thunk = builder.getOrCreateFunction(
       derivativeFnDecl, name, linkage, derivativeFnTy, IsBare, IsNotTransparent,
       derivativeFnDeclRef.isSerialized(), IsNotDynamic, ProfileCounter(),