[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