Add a comment explaining divergence with upstream

clang/lib/AST/Decl.cpp

@@ -3745,7 +3745,13 @@ bool FunctionDecl::doesDeclarationForceExternallyVisibleDefinition() const {
 
 FunctionTypeLoc FunctionDecl::getFunctionTypeLoc() const {
   const TypeSourceInfo *TSI = getTypeSourceInfo();
-
+  // CHERIOT: The following code diverges from upstream. The previous
+  // code returned a null FunctionTypeLoc when the function was
+  // annotated (impacting getReturnTypeSourceRange too). It is
+  // necessary to provide hints to replace the return type of a
+  // compartment entry point which returns void instead of int.
+  //
+  // https://github.com/llvm/llvm-project/pull/118420
   if (!TSI)
     return FunctionTypeLoc();