Add a new 'AddSignalHandler' function to Signals.h that allows

arbitrary functions to be run when a crash happens.  Delete 
RemoveDirectoryOnSignal as it is dead and has never had clients.

Change PrintStackTraceOnErrorSignal to be implemented in terms of
AddSignalHandler.

I updated the Win32 versions of these APIs, but can't test them.
If there are any problems, I'd be happy to fix them as well.

llvm-svn: 66072