Fix JS source maps for optional arguments

Also, when function has expression body, use expression instead
of function declaration to mark return statement.

js/js.translator/testData/lineNumbers/closure.kt

@@ -5,4 +5,4 @@ fun box(x: Int, y: Int): Int {
     return foo(y)
 }
 
-// LINES: 2 2 4 2 5
+// LINES: 2 4 2 5

js/js.translator/testData/lineNumbers/complexExpressionAsDefaultArgument.kt

@@ -0,0 +1,14 @@
+fun foo(
+        a: Int =
+            when (baz()) {
+                1 -> bar();
+                else -> 0
+            }
+): Int =
+        a + 1
+
+fun baz() = 1
+
+fun bar() = 2
+
+// LINES: 2 3 4 4 5 2 8 10 12

js/js.translator/testData/lineNumbers/coroutine.kt

@@ -14,4 +14,4 @@ suspend fun bar(): Unit {
     println(a + b)
 }
 
-// LINES: 4 5 4 6 4 9 9 9 9 9 9 9 9 * 10 11 11 11 11 11 * 11 12 13 13 13 13 13 13 14
+// LINES: 4 5 5 6 4 9 9 9 9 9 9 9 9 * 10 11 11 11 11 11 * 11 12 13 13 13 13 13 13 14

js/js.translator/testData/lineNumbers/expressionAsFunctionBody.kt

@@ -4,4 +4,4 @@ fun box() =
 fun foo() =
         23
 
-// LINES: 1 2 4 5
+// LINES: 2 5

js/js.translator/testData/lineNumbers/optionalArgs.kt

@@ -7,4 +7,4 @@ fun box(
     println(y)
 }
 
-// LINES: 2 2 3 4 6 7
+// LINES: 2 2 3 3 4 6 7