Fix accessor generation for suspend functions

1. JVM view of suspend accessors must be also an instance of AccessorForFunctionDescriptor,
thus `createSubstitutedCopy` should be correctly overidden.

2. accessibleFunctionDescriptor should unwap the initial suspend descriptor,
for the same reasons as for type aliases constructors and etc:
these descriptors are used as keys of the map of accessors, so
to avoid duplication for suspend view and initial suspend descriptor,
we always use the latter one as a key.

 #KT-15907 Fixed
 #KT-15935 Fixed

compiler/testData/codegen/box/coroutines/suspendFunctionAsCoroutine/privateFunctions.kt

@@ -0,0 +1,43 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+import kotlin.coroutines.*
+import kotlin.coroutines.intrinsics.*
+
+class X {
+    var res = ""
+    suspend fun execute() {
+        a()
+        b()
+    }
+
+    private suspend fun a() {
+        res += suspendThere("O")
+        res += suspendThere("K")
+    }
+
+    private suspend fun b() {
+        res += suspendThere("5")
+        res += suspendThere("6")
+    }
+}
+
+suspend fun suspendThere(v: String): String = suspendCoroutineOrReturn { x ->
+    x.resume(v)
+    SUSPENDED_MARKER
+}
+
+fun builder(c: suspend X.() -> Unit) {
+    c.startCoroutine(X(), EmptyContinuation)
+}
+
+fun box(): String {
+    var result = ""
+    builder {
+        execute()
+        result = res
+    }
+
+    if (result != "OK56") return "fail 1: $result"
+
+    return "OK"
+}

compiler/testData/codegen/box/coroutines/suspendFunctionAsCoroutine/privateInFile.kt

@@ -0,0 +1,30 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+import kotlin.coroutines.*
+import kotlin.coroutines.intrinsics.*
+
+
+var x = true
+private suspend fun foo(): String  {
+    if (x) {
+        return suspendCoroutineOrReturn<String> {
+            it.resume("OK")
+            SUSPENDED_MARKER
+        }
+    }
+
+    return "fail"
+}
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+fun box(): String {
+    var res = ""
+    builder {
+        res = foo()
+    }
+
+    return res
+}