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

[Mészáros Gergely via cfe-commits]

================
@@ -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}}
+};
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Maetveis wrote:

Its not relevant that the field is const, and the constructor is also irrelevant to the bug in question.

```suggestion
class HasUnusedField {
  int unused_; // expected-warning{{private field 'unused_' is not used}}
};
```

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