Casting to constrained existential in correct availability context fails at link time on older OSs

[AnthonyLatsis]

Running the following code on macOS 12.6.3 (no runtime support for constrained existentials) fails with JIT session error: Symbols not found: [ _swift_getExtendedExistentialTypeMetadata_unique ].

protocol P<T> {
  associatedtype T
}

public func foo<Bound>(x: Any, _: Bound.Type) {
  if #available(macOS 13.0, *) {
    if x is any P<Bound> {
      return
    }
  }
}

Environment

• Swift version 5.9-dev (LLVM 6a04c4848990c70, Swift f2bac1a22939a4d)

Extracted from #64657.