[DWARF parser] Fix nasty memory corruption in .dwo files handling.

Forge a test case where llvm-symbolizer has to use external .dwo
file to produce the inlining information.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217270 91177308-0d34-0410-b5e6-96231b3b80d8