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

[beccadax]

• Explanation: Modifies objcImpl to support the approved @objc @implementation syntax, ban resilient stored properties that cannot be supported by the current implementation, and remove its experimental status.
• Scope: Affects projects using objcImpl with both the new and old syntax.
• Issue: rdar://126948464&129178360&129247349&129225596&130707698
• Original PR: Handle resilient stored properties in objcImpl #73128, Change objcImpl syntax to @objc @implementation #73309, Fix two objcImpl resyntaxing bugs #74135, Soften min version for objcImpl early adopters #74161, Enable @objc @implementation (SE-0436) as an ordinary language feature #74801
• Risk: This has been qualified on main; there, a handful of projects that used @_objcImplementation needed minor source changes (e.g. moving declarations to different extensions, explicitly adding attributes). Other than those, I think the risk of unexpected regressions is low.
• Testing: Includes new unit tests. As mentioned, most of this PR has been qualified on main.
• Reviewer: @tshortli or @nkcsgexi, depending on the commit

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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.

[beccadax]

@swift-ci please test