[CommandLine] Allow Options to specify multiple OptionCategory's.

Summary:
It's not uncommon for separate components to share common
Options, e.g., it's common for related Passes to share Options in
addition to the Pass specific ones.

With this change, components can use OptionCategory's to simply help
output even if some of the options are shared.

Reviewed By: MaskRay

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61574

llvm-svn: 360179