Support suspend functions as superinterfaces

Forbid mixing suspend and non-suspend functional supertypes.
Since JVM BE generates suspend functional types as non-suspend ones
with SuspendFunction marker interface, there is not way to distinguish
non-suspend functional type from suspend one if they are mixed.
 #KT-18707 Fixed

compiler/testData/codegen/box/coroutines/suspendFunctionAsSupertype.kt

@@ -0,0 +1,31 @@
+// WITH_RUNTIME
+// IGNORE_BACKEND: JS, JS_IR, JVM
+// !LANGUAGE: +SuspendFunctionAsSupertype
+
+import kotlin.coroutines.*
+
+var res = "FAIL"
+
+class C: suspend () -> Unit {
+    override suspend fun invoke() {
+        res = "OK"
+    }
+}
+
+fun box(): String {
+    val o: suspend () -> Unit = object : suspend () -> Unit {
+        override suspend fun invoke() {
+            res = "OK"
+        }
+    }
+    o.startCoroutine(Continuation(EmptyCoroutineContext) {
+        it.getOrThrow()
+    })
+    if (res != "OK") return "FAIL 1: $res"
+
+    res = "FAIL 2"
+    C().startCoroutine(Continuation(EmptyCoroutineContext) {
+        it.getOrThrow()
+    })
+    return res
+}