[lldb][Test] Prevent generating DW_AT_location for unused argument

This test simply checks whether we can print an optimized
function argument. With recent changes to Clang the assumption
that we don't generate a `DW_AT_location` attribute for the
unused funciton parameter breaks.

This patch tries harder to get Clang to drop the location
from DWARF by making it generate an `undef` for `unused1`.
Drop the check for `unused2` since it adds no benefit.

Differential Revision: https://reviews.llvm.org/D132635