[JVM_IR]: Improve stepping for when.

Additionally, use the line number of the class for default interface
dispatch methods.

compiler/testData/debug/stepping/whenMultiLineSubject.kt

@@ -0,0 +1,56 @@
+// FILE: test.kt
+
+fun foo(x: Int): Int {
+    when (x) {
+        21 ->
+            1
+        42 ->
+            2
+        else ->
+            3
+    }
+
+    val t = when (x) {
+        21 ->
+            1
+        42 ->
+            2
+        else ->
+            3
+    }
+
+    return t
+}
+
+fun box() {
+    foo(21)
+    foo(42)
+    foo(63)
+}
+
+// LINENUMBERS
+// test.kt:26 box
+// test.kt:4 foo
+// test.kt:6 foo
+// test.kt:13 foo
+// test.kt:15 foo
+// test.kt:13 foo
+// test.kt:22 foo
+// test.kt:26 box
+// test.kt:27 box
+// test.kt:4 foo
+// test.kt:8 foo
+// test.kt:13 foo
+// test.kt:17 foo
+// test.kt:13 foo
+// test.kt:22 foo
+// test.kt:27 box
+// test.kt:28 box
+// test.kt:4 foo
+// test.kt:10 foo
+// test.kt:13 foo
+// test.kt:19 foo
+// test.kt:13 foo
+// test.kt:22 foo
+// test.kt:28 box
+// test.kt:29 box