[ELF] - Add a test case for relocation pointing to deduplicated COMDAT.

ELF spec doesn't allow a relocation to point to a deduplicated
COMDAT section. Unfortunately this happens in practice (e.g. .eh_frame)

We have a code in MarkLive.cpp for that and it was uncovered by any test case:
https://github.com/llvm-mirror/lld/blob/master/ELF/MarkLive.cpp#L199

Patch adds a test.

llvm-svn: 336259