[cxx-interop] Fix libc++ tests
These two tests require execution privileges in order to run `check-libcxx-version`, which is used to restrict the tests to a range of libc++ versions. They were failing on `non_executable` CI jobs because of missing `// REQUIRES: executable_test`.
rdar://145821727
(cherry picked from commit 8ef468c5f14bfbdac95f673ae818cdf99a1b6b83)