[Docs] Add note about legacy PM to Ch4 of tutorial

[Docs] Add note about legacy PM to Ch4 of tutorial

Add a note about legacy FunctionPassManager to the LLVM tutorial.

It seems to confuse some people, 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.

Differential Revision: https://reviews.llvm.org/D59258

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355930 91177308-0d34-0410-b5e6-96231b3b80d8