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

* 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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2383 91177308-0d34-0410-b5e6-96231b3b80d8