[K/JS] Reproduce issue with simple suspend functions that return Unit

^KT-79359

Author: JSMonk

analysis/low-level-api-fir/tests-gen/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLBlackBoxTestGenerated.java

@@ -0,0 +1,68 @@
+// KT-79359
+// TARGET_BACKEND: JS_IR
+// TARGET_BACKEND: JS_IR_ES6
+// IGNORE_BACKEND: JS_IR_ES6
+// WITH_STDLIB
+// WITH_COROUTINES
+
+import helpers.*
+import kotlin.coroutines.*
+import kotlin.coroutines.intrinsics.*
+
+var effects = ""
+
+private fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+suspend fun suspendHere(): Unit = suspendCoroutineUninterceptedOrReturn { x ->
+    x.resume(Unit)
+    COROUTINE_SUSPENDED
+}
+
+suspend fun simpleReturnsUnit() {
+    effects += "[simpleReturnsUnit]"
+}
+
+suspend fun callTailReturnsUnit() {
+    effects += "[callTailReturnsUnit]"
+    suspendHere()
+}
+
+suspend fun complexReturnsUnit(shouldSuspend: Boolean) {
+   if (shouldSuspend)  {
+       suspendHere()
+   }
+
+   effects += "[complexReturnsUnit]"
+}
+
+fun box(): String {
+    var failReason: String? = null
+
+    builder {
+        val simpleUnit = simpleReturnsUnit()
+        if (simpleUnit.toString() != "kotlin.Unit") {
+            failReason = "simpleReturnsUnit returns not Unit, but $simpleUnit"
+            return@builder
+        }
+
+        val tailcallUnit = callTailReturnsUnit()
+        if (tailcallUnit.toString() != "kotlin.Unit") {
+            failReason = "callTailReturnsUnit returns not Unit, but $tailcallUnit"
+            return@builder
+        }
+
+        val complexUnit = complexReturnsUnit(shouldSuspend = true)
+        if (complexUnit.toString() != "kotlin.Unit") {
+            failReason = "complexReturnsUnit returns not Unit, but $complexUnit"
+            return@builder
+        }
+    }
+
+    return when {
+        failReason != null -> failReason
+        effects != "[simpleReturnsUnit][callTailReturnsUnit][complexReturnsUnit]" -> "Fail: effects are $effects"
+        else -> "OK"
+    }
+}