[JS IR BE] Add Unit return type to function signature

js/js.translator/testData/box/coercion/bridgeChar.kt

@@ -4,6 +4,7 @@ open class A {
 }
 
 interface I {
+    @JsName("foo")
     fun foo(): Any
 }
 

js/js.translator/testData/box/coercion/derivedFunctionReturningUnit.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JS_IR
 // EXPECTED_REACHABLE_NODES: 1292
 var log = ""
 

js/js.translator/testData/box/dynamic/_common.kt

@@ -11,5 +11,6 @@ object t {
 }
 
 object n {
+    @JsName("valueOf")
     fun valueOf() = 42
 }

js/js.translator/testData/box/jsCode/tryCatchFinally.kt

@@ -4,6 +4,7 @@ package foo
 class Counter {
     var count: Int = 0
 
+    @JsName("inc")
     public fun inc() {
         count++
     }

js/js.translator/testData/box/trait/checkImplementationCharacteristics.kt

@@ -1,10 +1,12 @@
 // EXPECTED_REACHABLE_NODES: 1301
 
 public interface A {
+    @JsName("foo")
     fun foo() {
     }
 }
 public interface B : A {
+    @JsName("boo")
     fun boo() {
     }
 }