Add ifdef for AIX/MVS

Author: alisonzhang4

llvm/lib/Support/Unix/Path.inc

@@ -1024,13 +1024,17 @@ std::error_code openFile(const Twine &Name, int &ResultFD,
   auto Open = [&]() { return ::open(P.begin(), OpenFlags, Mode); };
   if ((ResultFD = sys::RetryAfterSignal(-1, Open)) < 0)
     return errnoAsErrorCode();
-  if (Access == FA_Read) {
-    struct stat Status;
-    if (fstat(ResultFD, &Status) == -1)
-      return errnoAsErrorCode();
-    if (S_ISDIR(Status.st_mode))
-      return make_error_code(errc::is_a_directory);
-  }
+  // The underlying operation on these platforms allow opening directories 
+  // for reading in more cases than other platforms. 
+  #if defined(__MVS__) || defined(_AIX)        
+    if (Access == FA_Read) {
+      struct stat Status;
+      if (fstat(ResultFD, &Status) == -1)
+        return errnoAsErrorCode();
+      if (S_ISDIR(Status.st_mode))
+        return make_error_code(errc::is_a_directory);
+    }
+  #endif  
 #ifndef O_CLOEXEC
   if (!(Flags & OF_ChildInherit)) {
     int r = fcntl(ResultFD, F_SETFD, FD_CLOEXEC);