[FIR] Properly set isOperator flag for java functions

^KT-56875 Fixed

compiler/fir/analysis-tests/testData/resolve/callResolution/incorrectPlusOperatorFromJava.fir.txt

@@ -0,0 +1,20 @@
+FILE: main.kt
+    public final operator fun R|Base|.plus(s: R|kotlin/String|): R|kotlin/Int| {
+        ^plus Int(0)
+    }
+    public final class Derived : R|Base| {
+        public constructor(): R|Derived| {
+            super<R|Base|>()
+        }
+
+        public final fun test_1(x: R|Base|): R|kotlin/Unit| {
+            lval y: R|kotlin/Int| = R|<local>/x|.R|/plus|(String())
+            R|<local>/y|.R|kotlin/Int.inc|()
+        }
+
+        public final fun test_2(x: R|Base|): R|kotlin/Unit| {
+            lval y: R|kotlin/String!| = R|<local>/x|.R|/Base.plus|(vararg(String()))
+            R|<local>/y|.R|kotlin/String.length|
+        }
+
+    }

compiler/fir/analysis-tests/testData/resolve/callResolution/incorrectPlusOperatorFromJava.kt

@@ -0,0 +1,22 @@
+// ISSUE: KT-56875
+// FILE: Base.java
+public class Base {
+    public String plus(String... strings) { // (1)
+        return "";
+    }
+}
+
+// FILE: main.kt
+operator fun Base.plus(s: String): Int = 0 // (2)
+
+class Derived : Base() {
+    fun test_1(x: Base) {
+        val y = x + "" // should resolve to (2)
+        y.inc() // should be ok
+    }
+
+    fun test_2(x: Base) {
+        val y = x.plus("") // should resolve to (1)
+        y.length // should be ok
+    }
+}