[llvm] [llvm-strings] Add --encoding & --unicode options (PR #221794)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

> Oh, also, I haven't looked at how intertwined the implementation is, but it might be worth considering how to split this up into multiple smaller PRs in a stack of some variety.

Okay, I'd hoped it was still small enough to be reviewable in one go but I'll split it up further. Because of the way GitHub works, I cannot use this as a base for subsequent PRs, so I've updated this PR to limit it to `--encoding={s,S,u}` to make it more reviewable and I'll prepare a stack offline. If this gets merged, I can open subsequent PRs.

> Please update the GNU strings command guide with the new options.

Done.

> I'd have expected to see new test files that are dedicated to the behaviour of the new options.

Okay, new test added for the different values of `--encoding`. I've still kept in the changes to the other tests as well because those were specifically testing how the encoding interacts with what these tests were testing.

> Please include a link to the RFC discussion in the PR description.

Will do. I need to also update the description to reflect what options are no longer in this PR, I will do that soon.

https://github.com/llvm/llvm-project/pull/221794