Previously, attempting to destroy a view with listeners more than once
throws an error during event listener cleanup. This happens because
`cleanup` field on the `TView` has already been cleared out by the time
the second destruction runs.
The `destroyed` flag on LView was previously being set in the `destroyLView` function,
but this flag was never _checked_ anywhere in the codebase. This commit
moves _setting_ this flag to the `cleanupView` function, just before
destroy hooks are called. This is necessary because the destroy hooks
can contain arbitrary user code, such as (surprise!) attempting to
destroy the view (again). We also add a check to `destroyLView` to skip
already-destroyed views. This prevents the cleanup code path from running twice.
PR Close#28413
Jeremy Elbourn
35e45dc894