Key decisions about 'bool' vs '_Bool' to be based on a new flag in langoptions.

This is simple enough, but then I thought it would be nice to make PrintingPolicy
get a LangOptions so that various things can key off "bool" and "C++" independently.
This spiraled out of control.  There are many fixme's, but I think things are slightly
better than they were before.

One thing that can be improved: CFG should probably have an ASTContext pointer in it,
which would simplify its clients.

llvm-svn: 74493