docs: Add HowToSetUpLLVMStyleRTTI.rst.

This document describes how to set up LLVM-style RTTI for a class
hierarchy. Surprisingly, this was not previously documented.

Also, link it into ProgrammersManual.html.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165293 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed