Selectively revert 36683a804c to resolve
a source compatibility regression. See inline comment for use case. We
are going to consider acknowledging this use case in the rules in a
future release.
https://github.com/swiftlang/swift/pull/72659 turned out to have some
source compatibility fallout that we need to fix. Instead of introducing
yet another brittle compatibility hack, stop emitting errors about a
missing `any` altogether until a future language mode.
Besides resolving the compatibility issue, this will encourage
developers to adopt any sooner and grant us ample time to gracefully
address any remaining bugs before the source compatibility burden
resurfaces.
A subsequent commit adds a diagnostic group that will allow users to
escalate these warnings to errors with `-Werror ExistentialAny`.
The non-metatype case was never supported. The same should hold for the
existential metatype case, which used to miscompile and now crashes
because the invariant reference is deemed OK but the erasure expectedly
fails to handle it:
```swift
class C<T> {}
protocol P {
associatedtype A
func f() -> any P & C<A>
func fMeta() -> any (P & C<A>).Type
}
do {
let p: any P
let _ = p.f() // error
let _ = p.fMeta() // crash
}
```
