[Typechecker] Add a comment to explain why we return SelfAccessKind::Mutating if decl is in a class-constrained protocol extension

Author: theblixguy

lib/Sema/TypeCheckDecl.cpp

@@ -2091,6 +2091,9 @@ llvm::Expected<SelfAccessKind>
 SelfAccessKindRequest::evaluate(Evaluator &evaluator, FuncDecl *FD) const {
   if (FD->getAttrs().getAttribute<MutatingAttr>(true)) {
     if (!FD->isInstanceMember() || !FD->getDeclContext()->hasValueSemantics()) {
+      // If this decl is on a class-constrained protocol extension, then
+      // the respect the explicit mutatingness. Otherwise, we would throw an
+      // error and break source compatibility.
       if (FD->getDeclContext()->isClassConstrainedProtocolExtension())
         return SelfAccessKind::Mutating;
       return SelfAccessKind::NonMutating;