Media: Migrate the data from the attachment post of the parent image when...

Media: Migrate the data from the attachment post of the parent image when saving an edited image. Copy:
- `post_title`,
- `post_content` (image description),
- `post_excerpt` (image caption as saved in the DB),
- `_wp_attachment_image_alt` meta (alt text for the img tag as saved in the DB).

Props spacedmonkey, joedolson, TimothyBlynJacobs, azaozz.
Fixes #50675.
Built from https://develop.svn.wordpress.org/trunk@48510


git-svn-id: https://core.svn.wordpress.org/trunk@48272 1a063a9b-81f0-0310-95a4-ce76da25c4cd