[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@359909
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