Expand on test comment why the new test case is required

Author: Maetveis

clang/test/SemaCXX/warn-unused-private-field.cpp

@@ -21,14 +21,20 @@ class SpaceShipDefaultCompare {
 };
 
 class EqDefaultCompareOutOfClass {
-  int used; // no warning
+  int used; // no warning, the compiler generated AST for the comparison operator
+            // references the fields of the class, and this should be considered
+            // a use.
+            // This test case is needed because clang does not emit the body
+            // of the defaulted operator when it is defined in-class until it
+            // finds a call to it. `-Wunused-private-field` is suppressed in
+            // a different way in that case.
   bool operator==(const EqDefaultCompareOutOfClass &) const;
 };
 
 bool EqDefaultCompareOutOfClass::operator==(const EqDefaultCompareOutOfClass &) const = default;
 
 class FriendEqDefaultCompareOutOfClass {
-  int used; // no warning
+  int used; // no warning, same reasoning just tested via a friend declaration.
   friend bool operator==(const FriendEqDefaultCompareOutOfClass &, const FriendEqDefaultCompareOutOfClass &);
 };