DwarfDebug class could generate the same (inlined) DIVariable twice:
1) when trying to find abstract debug variable for a concrete inlined instance.
2) when explicitly collecting info for variables that were optimized out.
This change makes sure that this duplication won't happen and makes
Clang pass "gdb.opt/inline-locals" test from gdb testsuite.
Reviewed by Eric Christopher.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159811 91177308-0d34-0410-b5e6-96231b3b80d8
