[debuginfo-tests] Un-XFAIL no passing unused-merged-value.c test

This test would previously expect one of the parameters to have
an incorrect DW_AT_location. Stepping through `fun` with a debugger
would then no reflect updates to one of the parameters.

With a recent change to Clang's DeadArgumentEliminationPass
(see `879f5118`) the generated
DWARF does not contain a location for `parama`, and stepping through
the function with `lldb` works as expected.

Differential Revision: https://reviews.llvm.org/D132664