Add synthetic flag to generated private suspend functions

Private suspend functions need to be generated as package-local, since
they are called from their continuations.
However, this means that they can be called from Java, which breaks
their private visibility.
Adding synthetic to them fixes the issue.
 #KT-17584: Fixed

compiler/testData/codegen/bytecodeListing/privateSuspendFun.kt

@@ -0,0 +1,5 @@
+private suspend fun foo() {}
+
+class A {
+    private suspend fun foo() {}
+}