* Add a stub to FunctionPass so that subclasses can declare that they do not

  modify the CFG.  It currently does nothing, but will in the future.
* Changes to make the public PassManager.h be MUCH smaller, and devoid of
  implementation details.  Now PassManager is just a Pimpl class that wraps
  PassManagerT<Module>, but lib/VMCore/Pass.cpp is now the only class that
  has to #include PassManagerT.h

llvm-svn: 2383