[Diagnostics][NFC] Add support for using the %select format specifier…

[owenv]

… with StringRef args

Get rid of some redundant availability diagnostics as a result.

Resolves SR-10588

[owenv]

cc @brentdax (original reporter)

[beccadax]

I’m on vacation for a few days, so I’ll tag myself for a review when I get back, but if someone else wants to do it before me, feel free!

[beccadax]

@swift-ci please smoke test

[jrose-apple]

Do we want to still call it %select or something else?

Can you update docs/Diagnostics.md too?

[owenv]

@jrose-apple Good idea, I added a note to the documentation.

I considered naming this something else and ultimately decided against it. Because the diagnostics format language is custom, I think it's better to keep the number of specifiers small and easy to remember. If this particular pattern was more widespread it might make more sense to introduce %if, %ifnonempty, or something similar.

[jrose-apple]

Looks good to me other than the comment about using it!

[jrose-apple]

@swift-ci Please smoke test

[jrose-apple]

Thanks, @owenv! I wonder if there are more diagnostics that could benefit from this.