Swift: fix extractor built with NDEBUG

[redsun82]

There was a call with side effects in an assert, that was therefore
not being called with NDEBUG turned on, changing extractor results.

[AlexDenisov]

We should be marking these with __used or similar, otherwise there are lots of "unused variable" warnings when built without assertions.

[redsun82]

yeah, I saw a lot of that as well, however maybe I would not do that before we roll out the shiny Diagnostic module that will save us all, as those warnings will probably go away anyway with that... We could maybe wrap asserts into a function maybe to avoid the warnings for now?

[AlexDenisov]

I think it's fine as it is right now, we can just wait for the diagnostics

[sashabu]

This is one of the motivating use cases for [[maybe_unused]] (and indeed the example at https://en.cppreference.com/w/cpp/language/attributes/maybe_unused).

And of course for more complex cases, another option is checking NDEBUG ourselves:

#ifdef NDEBUG
  bool unionCaseFound = 
#endif
      (... || fetchLabelFromUnionCase<Tag, Ts>(u, ret));
  assert(unionCaseFound && "llvm::PointerUnion not set to a known case");

Here, I think [[maybe_unused]] is the right "hammer": it's not very noisy, and does silence the warning (at the risk of leaving the unused variable if the assert goes away - but that's no worse a risk than learning to ignore warnings!). But checking NDEBUG can be useful when we don't want to run something in a release mode, but it doesn't comfortably fit inside an assert condition (e.g. if we'd written this as a loop instead of a fold).