[JS IR BE] Support main function

* Implement IR-based main function detector

js/js.translator/testData/box/main/incremental.kt

@@ -1,5 +1,4 @@
 // EXPECTED_REACHABLE_NODES: 1281
-// IGNORE_BACKEND: JS_IR
 // CALL_MAIN
 
 // FILE: ok.kt
@@ -19,6 +18,16 @@ fun main(args: Array<String>) {
     ok = "fail: b.b"
 }
 
+// FILE: 0.kt
+
+package b
+
+import ok.*
+
+fun main(args: Array<String>) {
+    ok = "fail: b"
+}
+
 // FILE: 2.kt
 
 package a

js/js.translator/testData/box/main/noArgs.kt

@@ -1,5 +1,4 @@
 // EXPECTED_REACHABLE_NODES: 1281
-// IGNORE_BACKEND: JS_IR
 // CALL_MAIN
 
 var ok: String = "fail"

js/js.translator/testData/box/main/simple.kt

@@ -1,5 +1,4 @@
 // EXPECTED_REACHABLE_NODES: 1281
-// IGNORE_BACKEND: JS_IR
 // CALL_MAIN
 
 var ok: String = "fail"

js/js.translator/testData/box/main/suspendMain.kt

@@ -1,5 +1,4 @@
 // EXPECTED_REACHABLE_NODES: 1296
-// IGNORE_BACKEND: JS_IR
 // CALL_MAIN
 
 import kotlin.coroutines.*

js/js.translator/testData/box/main/suspendMainNoArgs.kt

@@ -1,5 +1,4 @@
 // EXPECTED_REACHABLE_NODES: 1296
-// IGNORE_BACKEND: JS_IR
 // CALL_MAIN
 
 import kotlin.coroutines.*