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.txt

@@ -0,0 +1,10 @@
+@kotlin.Metadata
+public final class A {
+    public method <init>(): void
+    synthetic final @org.jetbrains.annotations.Nullable method foo(@org.jetbrains.annotations.Nullable p0: java.lang.Object): java.lang.Object
+}
+
+@kotlin.Metadata
+public final class PrivateSuspendFunKt {
+    synthetic final static @org.jetbrains.annotations.Nullable method foo(@org.jetbrains.annotations.Nullable p0: java.lang.Object): java.lang.Object
+}