add test to catch regressions in ABI stability of actor inits

it should not matter whether an initializer is delegating or not when
importing a library. this test helps make sure that stays true.

Author: kavon

test/Concurrency/Runtime/Inputs/MysteryInit.swift

@@ -0,0 +1,19 @@
+
+// This input is useful to ensure the delegation status of
+// an actor's initializer does not affect ABI stability
+public actor BigFoot {
+
+  public let name: String
+
+  private init(withName name: String) {
+    self.name = name
+  }
+
+  public init?() {
+  #if DELEGATES
+    self.init(withName: "Sasquatch")
+  #else
+    return nil
+  #endif
+  }
+}

test/Concurrency/Runtime/actor_init_abi.swift

@@ -0,0 +1,69 @@
+// RUN: %empty-directory(%t)
+
+// NOTE: the two tests below look like a lot of copy-pasting, but the only thing that's varying between
+// the two checks is whether "-D DELEGATES" is passed when compiling MysteryInit and whether the check-prefix when running the executable matches that or not.
+
+/// -----------------------------------------------------------------------
+/// Check if the actor init in the library can change from delegating to non-delegating
+/// -----------------------------------------------------------------------
+
+// ------> first library DOES delegate
+// RUN: %target-build-swift-dylib(%t/%target-library-name(MysteryInit)) -D DELEGATES -Xfrontend -disable-availability-checking -enable-library-evolution %S/Inputs/MysteryInit.swift -emit-module -emit-module-path %t/MysteryInit.swiftmodule -module-name MysteryInit
+// RUN: %target-codesign %t/%target-library-name(MysteryInit)
+
+
+// ------> make sure that works
+// RUN: %target-build-swift -parse-as-library  -Xfrontend -disable-availability-checking %s -lMysteryInit -I %t -L %t -o %t/main %target-rpath(%t)
+// RUN: %target-codesign %t/main
+// RUN: %target-run %t/main %t/%target-library-name(MysteryInit) | %FileCheck --check-prefix=CHECK-DELEGATES %s
+
+// ------> do a little internal sanity check on this test itself
+// RUN: %target-run %t/main %t/%target-library-name(MysteryInit) | not %FileCheck --check-prefix=CHECK-NO-DELEGATES %s
+
+// ------> now recompile that library's init so it DOES NOT delegate, without recompiling executable
+// RUN: %target-build-swift-dylib(%t/%target-library-name(MysteryInit)) -Xfrontend -disable-availability-checking -enable-library-evolution %S/Inputs/MysteryInit.swift -emit-module -emit-module-path %t/MysteryInit.swiftmodule -module-name MysteryInit
+// RUN: %target-codesign %t/%target-library-name(MysteryInit)
+
+// ------> re-run executable
+// RUN: %target-run %t/main %t/%target-library-name(MysteryInit) | %FileCheck --check-prefix=CHECK-NO-DELEGATES %s
+
+
+/// -----------------------------------------------------------------------
+/// now, other direction: Check if the actor init can change from non-delegating to delegating
+/// -----------------------------------------------------------------------
+
+// ------> library currently DOES NOT delegate. recompile executable to match.
+// RUN: %target-build-swift -parse-as-library  -Xfrontend -disable-availability-checking %s -lMysteryInit -I %t -L %t -o %t/main %target-rpath(%t)
+// RUN: %target-codesign %t/main
+// RUN: %target-run %t/main %t/%target-library-name(MysteryInit) | %FileCheck --check-prefix=CHECK-NO-DELEGATES %s
+
+// ------> now recompile that library's init so it DOES delegate, without recompiling executable
+// RUN: %target-build-swift-dylib(%t/%target-library-name(MysteryInit)) -D DELEGATES -Xfrontend -disable-availability-checking -enable-library-evolution %S/Inputs/MysteryInit.swift -emit-module -emit-module-path %t/MysteryInit.swiftmodule -module-name MysteryInit
+// RUN: %target-codesign %t/%target-library-name(MysteryInit)
+
+// ------> re-run executable
+// RUN: %target-run %t/main %t/%target-library-name(MysteryInit) | %FileCheck --check-prefix=CHECK-DELEGATES %s
+
+
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+
+// rdar://76038845
+// REQUIRES: concurrency_runtime
+// UNSUPPORTED: back_deployment_runtime
+
+import MysteryInit
+
+@main
+struct Main {
+  static func main() {
+    switch BigFoot() {
+    case .none:
+      print("bigfoot is myth")
+      // CHECK-NO-DELEGATES: bigfoot is myth
+    default:
+      print("bigfoot is real")
+      // CHECK-DELEGATES: bigfoot is real
+    }
+  }
+}