[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.

Specifically:

* 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

llvm-svn: 282482