[CommandLine] Change help output to prefix long options with `--` instead of `-`...

[CommandLine] Change help output to prefix long options with `--` instead of `-`. NFC . Part 3 of 5

Summary:
By default, `parseCommandLineOptions()` will accept either a
`-` or `--` prefix for long options -- options with names longer than
a single character.

While this change does not affect behavior, it will be helpful with a
subsequent change that requires long options use the `--` prefix.

Reviewers: rnk, thopre

Reviewed By: thopre

Subscribers: thopre, cfe-commits, hiraditya, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D61269

llvm-svn: 359909