[FIR LT] Add proper source for when with subject

#KT-56982 Fixed

compiler/testData/debug/stepping/whenExpr.kt

@@ -0,0 +1,30 @@
+// FILE: test.kt
+
+fun box() {
+    val a: Int? = 3
+    when (a) {
+        1 -> {
+            1
+        }
+        2 -> {
+            2
+        }
+        else -> {
+            0
+        }
+    }
+}
+
+// EXPECTATIONS JVM JVM_IR
+// test.kt:4 box
+// test.kt:5 box
+// test.kt:6 box
+// test.kt:9 box
+// test.kt:13 box
+// test.kt:16 box
+
+// EXPECTATIONS JS_IR
+// test.kt:4 box
+// test.kt:5 box
+// test.kt:5 box
+// test.kt:16 box