JVM_IR KT-42020 special IdSignature for some fake override members

compiler/testData/codegen/box/ir/clashingFakeOverrideSignatures.kt

@@ -0,0 +1,29 @@
+// TARGET_BACKEND: JVM
+// IGNORE_BACKEND_FIR: JVM_IR
+//  ^ TODO decide if we want to fix KT-42020 for FIR as well
+
+open class Base<T> {
+    fun foo(x: T) = "x:$x"
+    fun foo(y: String) = "y:$y"
+}
+
+open class Derived : Base<String>()
+
+fun box(): String {
+    val b = Base<String>()
+    val test1 = b.foo(x = "O") + b.foo(y = "K")
+    if (test1 != "x:Oy:K")
+        throw Exception("test1: $test1")
+
+    val d = Derived()
+    val test2 = d.foo(x = "O") + d.foo(y = "K")
+    if (test2 != "x:Oy:K")
+        throw Exception("test2: $test2")
+
+    val bd: Base<String> = Derived()
+    val test4 = bd.foo(x = "O") + bd.foo(y = "K")
+    if (test4 != "x:Oy:K")
+        throw Exception("test4: $test4")
+
+    return "OK"
+}

compiler/testData/codegen/box/ir/serializationRegressions/signatureClash.kt

@@ -3,7 +3,6 @@
 // IGNORE_BACKEND: JS
 // IGNORE_BACKEND: JS_IR
 // IGNORE_BACKEND: JS_IR_ES6
-// IGNORE_BACKEND: JVM_IR
 // IGNORE_BACKEND: NATIVE
 // IGNORE_BACKEND_FIR: JVM_IR
 // MODULE: lib