Adds using Swift from C++ example

examples/xplatform/swift_from_cxx/BUILD

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+load("//swift:swift_library.bzl", "swift_library")
+
+swift_library(
+    name = "landscape",
+    srcs = ["Landscape.swift"],
+    copts = ["-cxx-interoperability-mode=default"],
+    # Auto-generates landscape-Swift.h.
+    # It can be customized with generated_header_name.
+    generates_header = True,
+    module_name = "SwiftLandscape",
+)
+
+cc_binary(
+    name = "landscape_cxx_client",
+    srcs = ["landscape_cxx_client.cpp"],
+    deps = [":landscape"],
+)

examples/xplatform/swift_from_cxx/Landscape.swift

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+public struct MountainPeak {
+  let name: String
+  let height: Float
+
+  public init(name: String, height: Float) {
+    self.name = name
+    self.height = height
+  }
+
+  public func printDescription() {
+    print("Peak: \(name), height: \(height)")
+  }
+}
+
+public class MountainRange {
+  let peaks: [MountainPeak]
+
+  public init(peaks: [MountainPeak]) {
+    self.peaks = peaks
+  }
+
+  public func printPeaks() {
+    print("Peaks in range:")
+    for peak in peaks {
+      peak.printDescription()
+    }
+  }
+}
+
+// Simple enum.
+public enum VolcanoStatus {
+  case dormant
+  case active
+}
+
+// enum with associated values.
+public enum LandmarkIdentifier {
+  case name(String)
+  case id(Int)
+}

examples/xplatform/swift_from_cxx/landscape_cxx_client.cpp

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+#include <iostream>
+
+#include "examples/xplatform/swift_from_cxx/landscape-Swift.h"
+
+// The Swift module "SwiftLandscape" is imported as the C++ namespace
+// "SwiftLandscape".
+
+SwiftLandscape::VolcanoStatus invertVolcanoStatus(
+    SwiftLandscape::VolcanoStatus status) {
+  switch (status) {
+    case SwiftLandscape::VolcanoStatus::dormant:
+      // Returns `VolcanoStatus.active` case.
+      return SwiftLandscape::VolcanoStatus::active();
+    case SwiftLandscape::VolcanoStatus::active:
+      // Returns `VolcanoStatus.dormant` case.
+      return SwiftLandscape::VolcanoStatus::dormant();
+  }
+}
+
+void printLandmarkIdentifier(SwiftLandscape::LandmarkIdentifier identifier) {
+  switch (identifier) {
+    case SwiftLandscape::LandmarkIdentifier::name:
+      std::cout << (std::string)identifier.getName() << std::endl;
+      break;
+    case SwiftLandscape::LandmarkIdentifier::id:
+      std::cout << "unnamed landmark #" << identifier.getId() << std::endl;
+      break;
+  }
+}
+
+void structsAndClasses() {
+  std::cout << "----- Swift structs and classes in C++ -----" << std::endl;
+
+  // SwiftLandscape::MountainPeak is a Swift struct. But you must still use
+  // init(), because Swift initializers are not the same as C++
+  // constructors.
+  SwiftLandscape::MountainPeak mount_everest =
+      SwiftLandscape::MountainPeak::init("Mount Everest", 8848.0f);
+
+  // You can call a Swift struct member function from C++.
+  mount_everest.printDescription();
+
+  // Swift arrays are mapped to swift::Array<T> in C++.
+  auto peaks = swift::Array<SwiftLandscape::MountainPeak>::init();
+  {
+    SwiftLandscape::MountainPeak k2 =
+        SwiftLandscape::MountainPeak::init("K2", 8611.0f);
+    peaks.append(mount_everest);
+    peaks.append(k2);
+  }
+
+  SwiftLandscape::MountainRange himalayas =
+      SwiftLandscape::MountainRange::init(peaks);
+
+  himalayas.printPeaks();
+}
+
+void simpleEnums() {
+  std::cout << "----- Simple Swift enums -----" << std::endl;
+  auto dormant = SwiftLandscape::VolcanoStatus::dormant();
+  auto active = SwiftLandscape::VolcanoStatus::active();
+
+  std::cout << "dormant has C++ int value: "
+            << static_cast<int>(SwiftLandscape::VolcanoStatus::cases::dormant)
+            << std::endl;
+  std::cout << "active has C++ int value: "
+            << static_cast<int>(SwiftLandscape::VolcanoStatus::cases::active)
+            << std::endl;
+
+  auto inverted = invertVolcanoStatus(dormant);
+  std::cout << "inverted(dormant) == active? -> "
+            << ((inverted == active) ? "true" : "false") << std::endl;
+}
+
+void enumsWithAssociatedTypes() {
+  std::cout << "----- Swift enums with associated types -----" << std::endl;
+  auto new_landmark_id = SwiftLandscape::LandmarkIdentifier::id(1234);
+  printLandmarkIdentifier(new_landmark_id);
+
+  auto new_landmark_name =
+      SwiftLandscape::LandmarkIdentifier::name("Eiffel Tower");
+  printLandmarkIdentifier(new_landmark_name);
+}
+
+int main(int argc, char* argv[]) {
+  structsAndClasses();
+
+  simpleEnums();
+
+  enumsWithAssociatedTypes();
+
+  return 0;
+}