[Docs] Add a note about legacy FunctionPassManager to the LLVM tutorial.

[kristina]

Summary

It seems to confuse some people, I figured it may be worth adding a warning to the tutorial to elaborate and suggest using llvm::legacy::FunctionPassManager for now. Not a perfect solution but hopefully will avoid confusion in the meantime.

[philip.pfaffe]

LGTM. Did you verify that there's no conflicting info in the first three chapters of the tutorial?

[kristina]

I did, there are no references to any PassManager related material in the first three chapters.

llvm/trunk/docs/tutorial/LangImpl04.rst

 expressions (to make the add's lexically identical) and Common
 Subexpression Elimination (CSE) to delete the redundant add instruction.
 Fortunately, LLVM provides a broad range of optimizations that you can
 use, in the form of "passes".
 
 LLVM Optimization Passes
 ========================
 
+.. warning::
+
+   Due to the transition to the new PassManager infrastructure this tutorial
+   is based on ``llvm::legacy::FunctionPassManager`` which can be found in
+   `LegacyPassManager.h <http://llvm.org/doxygen/classllvm_1_1legacy_1_1FunctionPassManager.html>`_.
+   For the purpose of the this tutorial the above should be used until
+   the pass manager transition is complete.
+
 LLVM provides many optimization passes, which do many different sorts of
 things and have different tradeoffs. Unlike other systems, LLVM doesn't
 hold to the mistaken notion that one set of optimizations is right for
 all languages and for all situations. LLVM allows a compiler implementor
 to make complete decisions about what optimizations to use, in which
 order, and in what situation.
 
 As a concrete example, LLVM supports both "whole module" passes, which