[cxx-interop] [test] Refactor nonescapable test to use -verify

[j-hui]

Checking errors with FileCheck is fragile (because of line numbers) and difficult to follow.

[j-hui]

@swift-ci please smoke test

[j-hui]

@susmonteiro @Xazax-hun It might also be a good idea to split up this test. It's quite difficult to read, and a bit slow to run because of the dependency on std::optional and std::vector.

But I'll leave it up to y'all since you were the ones who actually wrote this test (-:

I just need this patch because my ongoing work elsewhere (#86048) apparently changes one of the error messages and I am having a really hard time telling whether the new diagnostic behavior should be expected.

[Xazax-hun]

Thanks a lot! This was long overdue and I wanted to get to it but always got distracted with something else. This is a huge improvement!

I have mixed feelings about splitting it up though. The more slightly related test files we have the harder to find the test case we are looking for or to figure out where to put new tests.

[susmonteiro]

Nice, thanks John!

[egorzhdan]

Thank you!

[j-hui]

ah yes, I forgot about Windows.... I just need %{fs-sep}

[j-hui]

@swift-ci please smoke test

[j-hui]

@swift-ci please smoke test windows platform