[runtime_cxxmodules] Export chrono file clock with std_filesystem module

Ensure _FilesystemClock is available with <filesystem>

Fixes the warning:
```
[79/2727] Generating G__Core.cxx, ../lib/Core.pcm
In module 'std' imported from input_line_1:1:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.4.sdk/usr/include/c++/v1/__chrono/time_point.h:33:52: error: definition of '_FilesystemClock' must be imported from module 'std.std_private_chrono_file_clock' before it is required
template <class _Clock, class _Duration = typename _Clock::duration>
                                                   ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.4.sdk/usr/include/c++/v1/__chrono/time_point.h:33:1: note: in instantiation of default argument for 'time_point<std::filesystem::_FilesystemClock>' required here
template <class _Clock, class _Duration = typename _Clock::duration>
^~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.4.sdk/usr/include/c++/v1/__chrono/file_clock.h:49:8: note: definition here is not reachable
struct _FilesystemClock {
```

Author: devajithvs

interpreter/cling/include/cling/std_darwin.modulemap

@@ -79,6 +79,8 @@ module std_expected [system] {
 }
 module std_filesystem [system] {
   header "filesystem"
+  // make sure the clock definition comes in together with <filesystem>
+  export std_private_chrono_file_clock
   export *
 }
 module std_format [system] {