Before the update to support the new syntax, the decl checker treated private and fileprivate members of objcImpl extensions as non-@objc by default. But SE-0436 specified that private and fileprivate members should be implicitly @objc unless opted out, so when support for the final syntax was added, this behavior was changed retroactively.
Unfortunately, we’ve found that some early adopters depended on the old behavior. Restore some logic deleted by #73309 for early adopter syntax *only*, with a warning telling the developer to put `final` or `@nonobjc` on the declaration if they want to preserve the behavior after updating it.
Also tweaks the ObjCImplementationChecker to ensure this logic will actually be run in time for it to suppress the “upgrade to @objc @implementation” warning, and corrects a couple of regressions arising from that change.
Fixes rdar://135747897.
In #69257, we modified `ObjCReason` to carry a pointer to the @implementation attribute for the `MemberOfObjCImplementationExtension` kind. This made it mark the @implementation attribute as invalid, suppressing diagnostics from the ObjCImplementationChecker.
However, invalidating the attribute *also* causes it to be skipped by serialization. That isn’t a problem if the diagnostics are errors, since we’ll never emit the serialized module, but #74135 softened these diagnostics to warnings for early adopters.
The upshot was that if Swift emitted one of these warnings when it compiled a library, clients of that library would see the objcImpl extension as a normal extension instead. This would cause various kinds of mischief: ambiguous name lookups because implementations weren’t being excluded, overrides failing because an implementation was `public` instead of `open`, asserts and crashes in SILGen and IRGen because stored properties were found in seemingly normal extensions, etc.
Fix this by setting a separate bit on ObjCImplementationAttr, rather than the invalid bit, and modifying the implementation checker to manually suppress many diagnostics when that bit is set.
Fixes rdar://134730183.
