Add Placement bit without removing others (#20398)

When scheduling a Placement effect, we should add the Placement bit
without resetting the others.

In the old fork, there are no flags to reset, anyway, since by the
time we reach the child reconciler, the flags will have already been
reset.

However, in the effects refactor, "static" flags are not reset, so this
can actually manifest as a bug. See #20285 for a regression test.

Author: acdlite