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
llvm-svn: 358428
b85f74a283