[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