[Sema] Replace invalid FIXME about memory leak. NFC

Added in my previous patch by mistake.

Author: ilya-biryukov

clang/lib/Sema/SemaConcept.cpp

@@ -341,7 +341,7 @@ bool Sema::CheckConstraintSatisfaction(
   OutSatisfaction = *Satisfaction;
   // We cannot use InsertPos here because CheckConstraintSatisfaction might have
   // invalidated it.
-  // FIXME: this leaks memory, we should allocate in the arena instead.
+  // Note that entries of SatisfactionCache are deleted in Sema's destructor.
   SatisfactionCache.InsertNode(Satisfaction.release());
   return false;
 }