DebugInfo: constrain gnu pubnames test further

Ensures that the pubnames entries actually refer to the intended
entities. This test could be more flexible if there was a way to do
multiline FileCheck matches with captures (in that way the test wouldn't
need to have hardcoded offset values and would thus be resilient to
changes in the layout of the DIEs in this CU).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191055 91177308-0d34-0410-b5e6-96231b3b80d8