[5.7][ASTPrinter] Print the desugared constraint type following the any keyword.

[hborla]

Cherry-pick of #59041

• Explanation: Currently, a protocol typealias with an underlying constraint type (i.e. a protocol or protocol composition) can be used as an existential type by accessing the alias through a type parameter, e.g.

protocol P1 {
  typealias A = P2
}

protocol P2 {}

func generic<T: P1>(value: (T) -> T.A) {} // okay

However, verifying that the underlying type of T.A is a constraint type is a circular computation in the general case, which is needed to use the any keyword. Resolving T.A where T is a type parameter needs a full generic signature to access the requirements on T, but a full generic signature needs the existential type verification.

This is an issue when T.A is written in source, and any is automatically printed in Swift interfaces. For now, simply print the underlying constraint type.

• Scope: This change only affects Swift interfaces for resilient libraries using existential types via typealiases in public API.
• Risk: Low.
• Testing: Added new unit tests.
• Reviewer: @slavapestov @tshortli

Resolves: rdar://93780361

[hborla]

@swift-ci please test