FIR: utilize checking subtype of functional type and finding invoke symbol

compiler/testData/codegen/box/coroutines/suspendConversion/subtypeOfFunctionalTypeToSuspendConversion.kt

@@ -15,12 +15,17 @@ fun builder(c: suspend () -> Unit) {
 
 suspend fun useSuspendFun(fn : suspend () -> String) = fn()
 suspend fun useSuspendFunInt(fn: suspend (Int) -> String) = fn(42)
+suspend fun useSuspendFunStringString(fn: suspend (String, String) -> String) = fn("O", "K")
 
-class Test : () -> String, (Int) -> String {
+open class Test : () -> String, (Int) -> String {
     override fun invoke(): String = "OKEmpty"
     override fun invoke(p: Int) = "OK$p"
 }
 
+class Sub : Test(), (String, String) -> String {
+    override fun invoke(p1: String, p2: String) = p1 + p2
+}
+
 fun box(): String {
     var test = "Failed"
     builder {
@@ -30,10 +35,14 @@ fun box(): String {
     if (test != "OKEmpty") return "failed 1"
 
     builder {
-        test = useSuspendFunInt(Test())
+        test = useSuspendFunInt(Sub())
     }
 
     if (test != "OK42") return "failed 2"
 
-    return "OK"
+    builder {
+        test = useSuspendFunStringString(Sub())
+    }
+
+    return test
 }