[cxx-interop] Fix Linux build with `SWIFT_ENABLE_EXPERIMENTAL_CXX_INTEROP=NO`
This fixes the Swift LSAN CI job Linux (https://ci.swift.org/job/oss-swift-RA-lsan-linux-ubuntu-18_04/) that started failing after https://github.com/apple/swift/pull/65398.
rdar://108674065
(cherry picked from commit b5866040ae7348515beebcc4e3cda167781439a4)