[SystemZ][ZOS] Guard using declaration for ::fchmodat

[zibi]

Summary

The use of fchmodat() is beeing guarded but its using declaration is not. Let's use the same guard in both places to avoid compiler errors on platforms where fchmodat does not exist.

[zibi]

ping - very minor change

[ldionne]

My preference would be to extend https://reviews.llvm.org/D90257 to handle those declarations too.

[zibi]

In D96303#2565665, @ldionne wrote:

My preference would be to extend https://reviews.llvm.org/D90257 to handle those declarations too.

I like the idea, @ldionne and agree. However, D90257 depends on D90188 and both did not land yet AFAIKS not to mention we will have to wait for the changes to be picked up by the build compiler.

Can we make this change now to unblock the build on z/OS?
We can always remove this change as part of the extension of D90257.

[ldionne]

I'm OK with that direction. But if you ship this, be aware that I'm going to remove your workaround when I ship the _LIBCPP_USING_IF_EXISTS patch, and that means you'll have to use a Clang that implements the attribute to get your build working. Are you OK with that?

[zibi]

In D96303#2569871, @ldionne wrote:

I'm OK with that direction. But if you ship this, be aware that I'm going to remove your workaround when I ship the _LIBCPP_USING_IF_EXISTS patch, and that means you'll have to use a Clang that implements the attribute to get your build working. Are you OK with that?

Yes, thank you.

libcxx/src/filesystem/posix_compat.h

int symlink_file(const char *oldname, const char *newname) {
   return ::symlink(oldname, newname);
 }
 int symlink_dir(const char *oldname, const char *newname) {
   return ::symlink(oldname, newname);
 }
 using ::chdir;
 using ::close;
 using ::fchmod;
+#if defined(AT_SYMLINK_NOFOLLOW) && defined(AT_FDCWD)
 using ::fchmodat;
+#endif
 using ::fstat;
 using ::ftruncate;
 using ::getcwd;
 using ::link;
 using ::lstat;
 using ::mkdir;
 using ::open;
 using ::readlink;