Add visible labels to inputs that are using placeholder attributes as a substitute for visible labeling.
Labels added or made visible on the customizer theme search, customizer widget search, customizer menu item search, customizer new page UI, the search plugins screens, the media search screens, and the classic editor link inserter.
Props afercia, joedolson, rcreators, sabernhardt.
See #40331.
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git-svn-id: http://core.svn.wordpress.org/trunk@57611 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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