[llvm] [llvm-strings] Add --encoding={s,S,utf8} option (PR #221794)

[James Henderson via llvm-commits]

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+## Show that llvm-strings uses the specified encoding.
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jh7370 wrote:

There does seem to be some inconsistency in these tests: negative-char.test uses `#` for the RUN/CHECK lines (so `##` is necessary for comments) but length.test doesn't. Honestly, I don't really care which approach is taken, nor do I really care about this being changed in existing tests. I was outlining the intended de facto policy, as I was the one who came up with the policy in the first place!

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