[CommandLine] Add callbacks to Options

Summary:
Add a new cl::callback attribute to Option.

This attribute specifies a callback function that is called when
an option is seen, and can be used to set other options, as in
option A implies option B.  If the option is a `cl::list`, and
`cl::CommaSeparated` is also specified, the callback will fire
once for each value.  This could be used to validate combinations
or selectively set other options.

Reviewers: beanz, thomasfinch, MaskRay, thopre, serge-sans-paille

Reviewed By: beanz

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70620