DebugInfo: Improve readability of test case added in r193878

The point is to ensure that the attribute in question
(DW_AT_data_member_location) is associated with the prior tag, so ensure
that we don't see another tag starting between the intended tag and the
desired attribute.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193884 91177308-0d34-0410-b5e6-96231b3b80d8