While creating FESM files, rollup usually drops all unused symbols.
All *__POST_NGCC__ are unused unless ngcc rewires stuff. To prevent this DCE
we reexport them as private symbols. If ngcc is not used, these symbols will
be dropped when we optimize an application bundle.
We don't have an infrastructure to test this fix, so I just manually inspected
the bundles before and after to verify that the fix works.
PR Close#25780