The type category enable * was implemented assuming a previous disable * had...

The type category enable * was implemented assuming a previous disable * had happened. While that will most likely be true in practice, the consequences of this not being the case will be a crash. I fix the crash by doing two things: 1) don't let already-enabled categories be enabled anyway; 2) if a category were disabled but with a bogus last-enabled position - highly highly unlikely - just put it in the first empty slot. I am not so sure 2) is bulletproof perfect, but I also don't think 2) will practically ever happen

llvm-svn: 219245