[Analyzer][solver][NFC] Add explanatory comments to trivial eq classes

[NoQ]

I understood this comment!

[martong]

In D106370#2892390, @NoQ wrote:

I understood this comment!

Thanks for the review!

clang/lib/StaticAnalyzer/Core/RangeConstraintManager.cpp

LLVM_NODISCARD static inline ProgramStateRef merge(RangeSet::Factory &F,
                                                      SymbolRef First,
                                                      SymbolRef Second);
   // Merge this class with the given class and return a new state.
   LLVM_NODISCARD inline ProgramStateRef
   merge(RangeSet::Factory &F, ProgramStateRef State, EquivalenceClass Other);
 
   /// Return a set of class members for the given state.
   LLVM_NODISCARD inline SymbolSet getClassMembers(ProgramStateRef State) const;
+
   /// Return true if the current class is trivial in the given state.
+  // A class is trivial if and only if there is not any member relations stored
+  // to it in State/ClassMembers.
+  // An equivalence class with one member might seem as it does not hold any
+  // meaningful information, i.e. that is a tautology. However, during the
+  // removal of dead symbols we do not remove classes with one member for
+  // resource and performance reasons. Consequently, a class with one member is
+  // not necessarily trivial. It could happen that we have a class with two
+  // members and then during the removal of dead symbols we remove one of its
+  // members. In this case, the class is still non-trivial (it still has the
+  // mappings in ClassMembers), even though it has only one member.
   LLVM_NODISCARD inline bool isTrivial(ProgramStateRef State) const;
+
   /// Return true if the current class is trivial and its only member is dead.
   LLVM_NODISCARD inline bool isTriviallyDead(ProgramStateRef State,
                                              SymbolReaper &Reaper) const;
 
   LLVM_NODISCARD static inline ProgramStateRef
   markDisequal(RangeSet::Factory &F, ProgramStateRef State, SymbolRef First,
                SymbolRef Second);
   LLVM_NODISCARD static inline ProgramStateRef