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.

llvm-svn: 159811