[FIR] Implement equals & hashCode for candidates

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/Candidate.kt

@@ -124,4 +124,19 @@ class Candidate(
         ExplicitReceiverKind.EXTENSION_RECEIVER, ExplicitReceiverKind.BOTH_RECEIVERS -> callInfo.explicitReceiver!!
         else -> implicitExtensionReceiverValue?.receiverExpression ?: FirNoReceiverExpression
     }
+
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (javaClass != other?.javaClass) return false
+
+        other as Candidate
+
+        if (symbol != other.symbol) return false
+
+        return true
+    }
+
+    override fun hashCode(): Int {
+        return symbol.hashCode()
+    }
 }