[DebugInfo][dexter] Tweak dexter test for merged values

Tweak dexter-tests/memvars/inline-escaping-function.c added in D94761
(b7e51620) by adding a 'param' use after the merge point. The test XFAILS
with and without this change, but without it the test looks very similar to
memvars/unused-merged-value.c. The test now demonstrates the problem more
clearly.