Clarify check_library_path semantics

[mhammond]

This library mode check is unclear. Every language has an identical check, except Swift which is a noop.

It looks like it's really just checking in a round-about way that library_mode::calc_cdylib_name didn't return None - but why trait and that fact Swift is the only thing that does not check? And in my local test, Swift doesn't fail if you do. I got a bit lost in blame, so @bendk do you have recollections about this?

[bendk]

Yeah, that's definitely unclear. My half memory / half guess is that Swift doesn't need the check because it can work with static linking.

[mhammond]

This was answered in #2094 (comment)