[DWARF] Fix dwarf5-index-is-used.cpp

Summary:
`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

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D51208

llvm-svn: 341296