[CommandLine] Add inline ArgName printing

Summary:
This patch adds PrintArgInline (after PrintArg) that strips the
leading spaces from an argument before printing them, for usage
inline.

Related bug: PR42943 <https://bugs.llvm.org/show_bug.cgi?id=42943>

Patch by Daan Sprenkels!

Reviewers: jhenderson, chandlerc, hintonda

Reviewed By: jhenderson

Subscribers: hiraditya, kristina, llvm-commits, dsprenkels

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69501