[DWARF] Fix dwarf5-index-is-used.cpp
dwarf5-index-is-used.cpp have been failing after rL340206, because clang
have stopped to emit pubnames by default after that change. Current patch adds
-gpubnames option to the clang command line in the test to emit pubnames.
Reviewers: labath, dblaikie
Reviewed By: labath
Subscribers: clayborg, probinson, teemperor, lldb-commits, aprantl, JDevlieghere, abidh, stella.stamenova
Differential Revision: https://reviews.llvm.org/D51208