CMake: Specify modern Cxx interop flag when build SwiftCompilerSources.

Now that swift-ci builds with a Swift 5.9 or later Swift toolchain, we can
specify the `-cxx-interoperability-mode` flag instead of the deprecated
`-enable-experimental-cxx-interop` flag.

Author: tshortli

SwiftCompilerSources/CMakeLists.txt

@@ -101,7 +101,7 @@ function(add_swift_compiler_modules_library name)
   set(swift_compile_options
       "-color-diagnostics"
       "-Xfrontend" "-validate-tbd-against-ir=none"
-      "-Xfrontend" "-enable-experimental-cxx-interop"
+      "-cxx-interoperability-mode=default"
       "-Xfrontend" "-disable-target-os-checking"
       "-Xcc" "-std=c++17"
       "-Xcc" "-DCOMPILED_WITH_SWIFT" "-Xcc" "-DSWIFT_TARGET"