[cxx-interop] pass -std=c++20 to configurations using std::span

When the benchmarks are built with SwiftPM, the -std=c++20 flag is passed if the -cxx-interoperability-mode is present. This patch switches from -Xfrontend -enable-experimental-cxx-interop to the required interoperability flag.

Author: susmonteiro

benchmark/Package.swift

@@ -127,8 +127,7 @@ targets.append(
     dependencies: swiftBenchDeps,
     path: "utils",
     sources: ["main.swift"],
-    swiftSettings: [.unsafeFlags(["-Xfrontend",
-                                  "-enable-experimental-cxx-interop",
+    swiftSettings: [.unsafeFlags(["-cxx-interoperability-mode=default",
                                   "-I",
                                   "utils/CxxTests"])]))
 
@@ -166,8 +165,7 @@ targets += cxxSingleSourceLibraries.map { name in
     dependencies: singleSourceDeps,
     path: "cxx-source",
     sources: ["\(name).swift"],
-    swiftSettings: [.unsafeFlags(["-Xfrontend",
-                                  "-enable-experimental-cxx-interop",
+    swiftSettings: [.unsafeFlags(["-cxx-interoperability-mode=default",
                                   "-I",
                                   "utils/CxxTests",
                                   // FIXME: https://github.com/apple/swift/issues/61453