Act on experimental C++ interoperability flags

- Pass appropriate options to fronend jobs.
- Use `clang++` as the linker driver when in experimental C++ interoperability mode

Author: artemcm

Sources/SwiftDriver/Jobs/DarwinToolchain+LinkerSupport.swift

@@ -303,6 +303,11 @@ extension DarwinToolchain {
       commandLine.appendFlag("-application_extension")
     }
 
+    // On Darwin, we only support libc++.
+    if parsedOptions.contains(.enableExperimentalCxxInterop) {
+      commandLine.appendFlag("-lc++")
+    }
+
     // inputs LinkFileList
     if shouldUseInputFileList {
       commandLine.appendFlag(.filelist)

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -91,6 +91,15 @@ extension Driver {
       commandLine.appendFlag(.disableObjcInterop)
     }
 
+    // Add flags for C++ interop
+    if parsedOptions.hasArgument(.enableExperimentalCxxInterop) {
+      commandLine.appendFlag(.enableCxxInterop)
+    }
+    if let stdlibVariant = parsedOptions.getLastArgument(.experimentalCxxStdlib)?.asSingle {
+      commandLine.appendFlag("-Xcc")
+      commandLine.appendFlag("-stdlib=\(stdlibVariant)")
+    }
+
     // Handle the CPU and its preferences.
     try commandLine.appendLast(.targetCpu, from: &parsedOptions)
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -3017,6 +3017,33 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
+  func testCXXInteropOptions() throws {
+    do {
+      var driver = try Driver(args: ["swiftc", "-enable-experimental-cxx-interop", "foo.swift"])
+      let plannedJobs = try driver.planBuild().removingAutolinkExtractJobs()
+      XCTAssertEqual(plannedJobs.count, 2)
+      let compileJob = plannedJobs[0]
+      let linkJob = plannedJobs[1]
+      XCTAssertTrue(compileJob.commandLine.contains(.flag("-enable-cxx-interop")))
+      if driver.targetTriple.isDarwin {
+        XCTAssertTrue(linkJob.commandLine.contains(.flag("-lc++")))
+      }
+    }
+    do {
+      var driver = try Driver(args: ["swiftc", "-enable-experimental-cxx-interop",
+                                     "-experimental-cxx-stdlib", "libc++", "foo.swift"])
+      let plannedJobs = try driver.planBuild().removingAutolinkExtractJobs()
+      XCTAssertEqual(plannedJobs.count, 2)
+      let compileJob = plannedJobs[0]
+      let linkJob = plannedJobs[1]
+      XCTAssertTrue(compileJob.commandLine.contains(.flag("-enable-cxx-interop")))
+      XCTAssertTrue(compileJob.commandLine.contains(.flag("-stdlib=libc++")))
+      if driver.targetTriple.isDarwin {
+        XCTAssertTrue(linkJob.commandLine.contains(.flag("-lc++")))
+      }
+    }
+  }
+
   func testVFSOverlay() throws {
     do {
       var driver = try Driver(args: ["swiftc", "-c", "-vfsoverlay", "overlay.yaml", "foo.swift"])