[Patch] Fix for PR9499 (confusing behavior of FastFoldingSetNode).

I've made a patch which provides one possible fix for the unintuitive
behavior of FastFoldingSetNode. I'm not sure whether that's a good way to
solve this. It's based largely on the documentation found in the
FastFoldingSet.h file, which says Profile functions should use "Add" instead
of simply setting their parameter.

Patch is attached on this mail and on the PR. Thanks!

fixes.patch (1.46 KB)