[docs] Make WritingAnLLVMPass.rst up-to-date with current state of things
This patch updates WritingAnLLVMPass.rst to make it in line with current state of things.
- Makefile instructions replaced with CMake ones
- Filenames replaced with correct ones
- Example reformatted a bit to make it less confusing and more conforming to LLVM Coding Standards
- opt tool output updated with what it actually prints nowdays
- "gcse" (which doesn't exist anymore) replaced with "gvn" (which still does)
Differential Revision: https://reviews.llvm.org/D24233