In my case, all warn disappeared after I directly changed swift version from 2.x to 4.0 in build settings except two warn.
These warning related to myprojectnameTests
and myprojectnameUITests
folder. I didn't notice and I thought its relate to direct immigration from Xcode 7 to Xcode 9 and I thought I couldn't solve this problem and I should install missed Xcode 8 version.
In my case, I deleted these folders and all warns disappeared, but you can recreate this folder and contains using this:
file > new > target > (uitest or unittest extensions)
and use this article for create test cases: https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/testing_with_xcode/chapters/04-writing_tests.html