[clang] [Clang] Fix -Wunused-private-field false negative with defaulted comparison operators (PR #116871)

[Mészáros Gergely via cfe-commits]

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@@ -20,6 +20,17 @@ class SpaceShipDefaultCompare {
   int operator<=>(const SpaceShipDefaultCompare &) const = default;
 };
 
+class UnusedConstPrivateField {
+ public:
+  UnusedConstPrivateField() : unused_(0) {}
+ private:
+  const int unused_; // expected-warning{{private field 'unused_' is not used}}
+};
+
+class FriendEqDefaultCompare {
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Maetveis wrote:

I would emphasize with the name that this class has no relation to the previous one, because that is what the bug is about.

```suggestion
class UnrelatedFriendEqDefaultCompare {
```

https://github.com/llvm/llvm-project/pull/116871