Move the `ID_TAG_ID_LINK_PLACEHOLDER` bit of `id->tag` to
`id->runtime.readfile_data->tags.is_id_link_placeholder`. It also
introduces the necessary stucts and allocation/freeing code.
Old code:
```cpp
if (id_tag & ID_TAG_ID_LINK_PLACEHOLDER) {
```
New code:
```cpp
if (readfile_id_runtime_tags(id).is_id_link_placeholder) {
```
where `readfile_id_runtime_tags(id)` is a getter for
`id->runtime.readfile_data->tags` that is null-safe for
`id->runtime.readfile_data`. The `readfile_data` is not allocated in
these cases:
1. When reading undo steps, because that doesn't have to deal with
versioning or linking (which are the sole purposes for this
struct).
2. When linking from another file (for example from the 'Link...'
operator). The just-linked IDs will have the `readfile_data`
struct, but already-loaded IDs will already have had those freed.
No functional changes intended.
Pull Request: https://projects.blender.org/blender/blender/pulls/132169
Design Task: #131695
2ec1b6887d