libXcodeDebuggerSupport.dylib is missing in iOS 4.2.1 development SDK
If you do not have anything to lose, just dump the whole Developer folder (or Developer4.2 or however you call it). Reinstall from scratch with the official release of iOS 4.2 SDK - "Xcode 3.2.5 and iOS SDK 4.2". I did that and found everything gets installed under 4.2 (8C134) only, and does not use 4.2.1 at all.
I do not have enough points to comment to the accepted answer.Trinca's solution works for me.
Only one addition though:I got a permission denied in Xterm when I tried to execute the ln command.
just had to enter
sudo xtermand enter my admin password for the systemand repeat the process in the new terminal.
I am wondering how many people out there are re-installing Xcode because of this issue.It is definitely a huge waste of time, if people went ahead with re-installation without checking for a better solution. Apple has to keep enough reviewers for sdk runtime checks, and not just for our apps :).