[CommandLine] Add long option flag for cl::ParseCommandLineOptions . Part 5 of 5

Summary:
If passed, the long option flag makes the CommandLine parser
mimic the behavior or GNU getopt_long.  Short options are a single
character prefixed by a single dash, and long options are multiple
characters prefixed by a double dash.

This patch was motivated by the discussion in the following thread:
http://lists.llvm.org/pipermail/llvm-dev/2019-April/131786.html

Reviewed By: MaskRay

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61294

llvm-svn: 360532