[CommandLineParser] Add DefaultOption flag

[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.

Reviewers: alexfh, klimek

Reviewed By: klimek

Subscribers: 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/cfe/trunk@358337 91177308-0d34-0410-b5e6-96231b3b80d8