On Mac, try use commandline tools first instead of 'open' for dot files

[ychen]

Summary

It's a hassle[1] to use 'open' on Mac to open dot files since it requires an application be installed. Better try commandline tools first, and resort to 'open' as the backup.

[1] brew does not provide graphviz app anymore.
https://github.com/Homebrew/homebrew-core/issues/31510

llvm/lib/Support/GraphWriter.cpp

 
 bool llvm::DisplayGraph(StringRef FilenameRef, bool wait,
                         GraphProgram::Name program) {
   std::string Filename = FilenameRef;
   std::string ErrMsg;
   std::string ViewerPath;
   GraphSession S;
 
-#ifdef __APPLE__
-  wait &= !ViewBackground;
-  if (S.TryFindProgram("open", ViewerPath)) {
-    std::vector<StringRef> args;
-    args.push_back(ViewerPath);
-    if (wait)
-      args.push_back("-W");
-    args.push_back(Filename);
-    errs() << "Trying 'open' program... ";
-    if (!ExecGraphViewer(ViewerPath, args, Filename, wait, ErrMsg))
-      return false;
-  }
-#endif
   if (S.TryFindProgram("xdg-open", ViewerPath)) {
     std::vector<StringRef> args;
     args.push_back(ViewerPath);
     args.push_back(Filename);
     errs() << "Trying 'xdg-open' program... ";
     if (!ExecGraphViewer(ViewerPath, args, Filename, wait, ErrMsg))
       return false;
   }
if (S.TryFindProgram("xdot|xdot.py", ViewerPath)) {
 
     args.push_back("-f");
     args.push_back(getProgramName(program));
 
     errs() << "Running 'xdot.py' program... ";
     return ExecGraphViewer(ViewerPath, args, Filename, wait, ErrMsg);
   }
 
+#ifdef __APPLE__
+  wait &= !ViewBackground;
+  if (S.TryFindProgram("open", ViewerPath)) {
+    std::vector<StringRef> args;
+    args.push_back(ViewerPath);
+    if (wait)
+      args.push_back("-W");
+    args.push_back(Filename);
+    errs() << "Trying 'open' program... ";
+    if (!ExecGraphViewer(ViewerPath, args, Filename, wait, ErrMsg))
+      return false;
+  }
+#endif
+
   enum ViewerKind {
     VK_None,
     VK_OSXOpen,
     VK_XDGOpen,
     VK_Ghostview,
     VK_CmdStart
   };
   ViewerKind Viewer = VK_None;