[FIR2IR] Properly create FIR f/o for lazy IR f/o

```kotlin
interface A {
    fun foo() // (1)
}

interface B : A {
    // f/o fun foo() // (2)
}
```

In previous commits it was forgotten to create new FIR declarations for
  IR fake-overrides, which led to the situation when `IR lazy functions
  of `(1)` and `(2)` both contained fir of function (1) as their base
  declaration

It seems this change fixed some cases from KT-42020

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

@@ -1,6 +1,6 @@
 // TARGET_BACKEND: JVM
-// IGNORE_BACKEND_K2: JVM_IR, JS_IR
-// FIR status: validation failed. TODO decide if we want to fix KT-42020 for FIR as well
+// IGNORE_CODEGEN_WITH_IR_FAKE_OVERRIDE_GENERATION
+// Reason: KT-42020
 
 open class Base<T> {
     fun foo(x: T) = "x:$x"
@@ -26,4 +26,4 @@ fun box(): String {
         throw Exception("test4: $test4")
 
     return "OK"
-}
+}