[release/6.0] ASTMangler: skip mangling Copyable dependents

[kavon]

• Explanation: The Swift 6.0 compiler can crash when trying to check if a returned-value conforms to the constraint of an opaque return type. It triggers when a conformance requirement is conditional on Copyable. This results in a regression from Swift 5.10, for example, in the following program:

protocol Marked {
  func announce()
}
extension Marked {
  func announce() {}
}

struct MyType<Wrapped> {}
extension MyType: Marked where Wrapped: Marked {}

extension Optional: Marked {}

func makeWrapper<P>(wrapping _: P.Type) -> some Marked {
    MyType<Optional<P>>()
}

• Scope: This is a regression where the compiler previously accepted the program, but cause of some adoption of ~Copyable in the stdlib, it will trip over the same code without this fix.
• Issue: rdar://135310019
• Original PR: ASTMangler: skip mangling Copyable dependents #76293 ASTMangler: sink fix to be more safe #76432
• Risk: Low. It's a simple, straightforward fix to skip the new Copyable requirement that has now appeared.
• Testing: Swift CI
• Reviewer: @slavapestov

[kavon]

@swift-ci test

[kavon]

@swift-ci test