[turbofan] Switch serializer hints to FunctionalSet

Bug: v8:9720
Change-Id: I681c521c7d368148d41aaa4a0694194e33d9a091
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1800578
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63824}

src/compiler/functional-list.h

@@ -90,6 +90,8 @@ class FunctionalList {
 
   size_t Size() const { return elements_ ? elements_->size : 0; }
 
+  void Clear() { elements_ = nullptr; }
+
   class iterator {
    public:
     explicit iterator(Cons* cur) : current_(cur) {}