🌸 Enable @objc @implementation in the form approved by SE-0436

[beccadax]

Cherry-picks #73128, #73309, #74135, #74161, and #74801 to release/6.0. In combination, these changes implement the approved design for SE-0436 and add it as a normal language feature. Note that with the exception of #74801 (which marks the feature non-experimental and updates diagnostic wording), these changes have been baking in main for 1-3 months.