[cxx-interop] Build benchmarks with modern C++ interop flag

To build benchmarks that use C++ `std::span`, we need to use C++20 standard.

SwiftPM supports `.interoperabilityMode(.Cxx)` build setting starting from version 5.9. In more recent versions, SwiftPM is also able to correctly propagate the C++ standard version (e.g. `-Xcc -std=c++20`) to the Swift compiler invocation, when C++ interop is enabled. The C++ standard version is only propagated if the modern build setting is used. This has caused Linux CI failures in the past.

This change switches the package manifest to use the modern build setting to enable C++ interop.

benchmark/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.4
+// swift-tools-version:5.9
 
 import PackageDescription
 import Foundation
@@ -127,11 +127,9 @@ targets.append(
     dependencies: swiftBenchDeps,
     path: "utils",
     sources: ["main.swift"],
-    swiftSettings: [.unsafeFlags(["-cxx-interoperability-mode=default",
-                                  "-I",
-                                  "utils/CxxTests",
-                                  // FIXME(rdar://136138941): these flags should be redundant because of cxxLanguageStandard
-                                  "-Xcc", "-std=c++20"])]))
+    swiftSettings: [.interoperabilityMode(.Cxx),
+                    .unsafeFlags(["-I",
+                                  "utils/CxxTests"])]))
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
 targets.append(
@@ -167,11 +165,9 @@ targets += cxxSingleSourceLibraries.map { name in
     dependencies: singleSourceDeps,
     path: "cxx-source",
     sources: ["\(name).swift"],
-    swiftSettings: [.unsafeFlags(["-cxx-interoperability-mode=default",
-                                  "-I",
+    swiftSettings: [.interoperabilityMode(.Cxx),
+                    .unsafeFlags(["-I",
                                   "utils/CxxTests",
-                                  // FIXME(rdar://136138941): these flags should be redundant because of cxxLanguageStandard
-                                  "-Xcc", "-std=c++20",
                                   // FIXME: https://github.com/apple/swift/issues/61453
                                   "-Xfrontend", "-validate-tbd-against-ir=none"])])
 }