[presets] Actually make https://github.com/swiftlang/swift/pull/81284 work

CMakeLists.txt

@@ -824,6 +824,9 @@ elseif(UNIX)
   include(UnixCompileRules)
 endif()
 
+# Add any extra C++ compilation options that were passed down.
+add_compile_options($<$<COMPILE_LANGUAGE:CXX>:${SWIFT_EXTRA_CXX_FLAGS}>)
+
 if(CMAKE_C_COMPILER_ID MATCHES Clang)
   add_compile_options($<$<OR:$<COMPILE_LANGUAGE:C>,$<COMPILE_LANGUAGE:CXX>>:-Werror=gnu>)
 endif()