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/backend/src/org/jetbrains/kotlin/codegen/AsmUtil.java

@@ -244,7 +244,8 @@ public class AsmUtil {
         int flags = getVisibilityAccessFlag(functionDescriptor);
         flags |= getVarargsFlag(functionDescriptor);
         flags |= getDeprecatedAccessFlag(functionDescriptor);
-        if (state.getDeprecationProvider().isDeprecatedHidden(functionDescriptor)) {
+        if (state.getDeprecationProvider().isDeprecatedHidden(functionDescriptor) ||
+            (functionDescriptor.isSuspend()) && functionDescriptor.getVisibility().equals(Visibilities.PRIVATE)) {
             flags |= ACC_SYNTHETIC;
         }
         return flags;