[ELF] - Add a test for --gc-sections --undefined=foo combination.

GC code contains following logic uncovered by tests:

for (StringRef S : Config->Undefined)
  MarkSymbol(Symtab->find(S));

As far I can tell we never had test for that before,
patch fixes it.

Differential revision: https://reviews.llvm.org/D36519

llvm-svn: 310556