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

Author: susmonteiro

benchmark/Package.swift

@@ -130,7 +130,9 @@ targets.append(
     swiftSettings: [.unsafeFlags(["-Xfrontend",
                                   "-enable-experimental-cxx-interop",
                                   "-I",
-                                  "utils/CxxTests"])]))
+                                  "utils/CxxTests",
+                                  "-Xcc",
+                                  "-std=c++20"])]))
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 targets.append(
@@ -170,6 +172,8 @@ targets += cxxSingleSourceLibraries.map { name in
                                   "-enable-experimental-cxx-interop",
                                   "-I",
                                   "utils/CxxTests",
+                                  "-Xcc",
+                                  "-std=c++20",
                                   // FIXME: https://github.com/apple/swift/issues/61453
                                   "-Xfrontend", "-validate-tbd-against-ir=none"])])
 }