Make ForSwiftFoundationOnly an explicit submodule that doesn't re-export

This allows us to prevent the ambiguity for various STATX defines from linux/stat.h when building for Android, as those are defined in the Android platform module

Author: hyp

CoreFoundation/Base.subproj/ForSwiftFoundationOnly.h

@@ -64,6 +64,7 @@
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <termios.h>
+#include <linux/stat.h>
 #elif TARGET_OS_WASI
 #include <fcntl.h>
 #include <sys/stat.h>

CoreFoundation/Base.subproj/module.modulemap

@@ -1,6 +1,12 @@
 framework module CoreFoundation [extern_c] [system] {
     umbrella header "CoreFoundation.h"
     explicit module CFPlugInCOM { header "CFPlugInCOM.h" }
+    explicit module ForSwiftFoundationOnly {
+      header "ForSwiftFoundationOnly.h"
+      // Do not re-export imported Clang modules to avoid pulling in
+      // system headers like linux/stat.h whose constants might conflict
+      // with constants from the platform module.
+    }
 
     export *
     module * {