Fix the logic in DynamicLoaderMacOSXDYLD::Clear that would only remove the old...

Fix the logic in DynamicLoaderMacOSXDYLD::Clear that would only remove the old dyld notification        
breakpoint only  if the process it was for is still alive.  We need to always remove this because
it has a pointer to the old loader, and    if we ever hit it we will crash.  I also put in a sanity
check in the callback function to make sure we don't invoke it if the process is wrong.

<rdar://problem/21006189>

llvm-svn: 237866