[FIR LT] Get correct startOffset value for declarations with comments

#KT-56913 Fixed
#KT-56926 Fixed

compiler/testData/debug/stepping/commentBeforeFunctionWithDefault.kt

@@ -0,0 +1,20 @@
+// FILE: test.kt
+
+// Comment before
+fun foo(i: Int = 1): Int {
+    return i
+}
+
+fun box() {
+    foo()
+}
+
+// EXPECTATIONS JVM JVM_IR
+// test.kt:9 box
+// test.kt:5 foo
+// test.kt:9 box
+// test.kt:10 box
+
+// EXPECTATIONS JS_IR
+// test.kt:9 box
+// test.kt:10 box