[TEST-ONLY] Fix IRGen/hermetic-seal-exec.swift
Fix IRGen/hermetic-seal-exec.swift after new driver switch. New driver
requires explicit `-lobjc` flag to link objc runtime. Most of the time,
this test doesn't need to link objc runtime because it disables
interop, but not when it runs on very old targets that uses very old
compatible static library.
rdar://173266785
