[CommandLineParser] Add DefaultOption flag
Summary: Add DefaultOption flag to CommandLineParser which provides a
default option or alias, but allows users to override it for some
other purpose as needed.
Also, add `-h` as a default alias to `-help`, which can be seamlessly
overridden by applications like llvm-objdump and llvm-readobj which
use `-h` as an alias for other options.
(relanding after revert, r358414)
Added DefaultOptions.clear() to reset().
Reviewers: alexfh, klimek
Reviewed By: klimek
Subscribers: kristina, MaskRay, mehdi_amini, inglorion, dexonsmith, hiraditya, llvm-commits, jhenderson, arphaman, cfe-commits
Tags: #clang, #llvm
Differential Revision: https://reviews.llvm.org/D59746
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358428
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