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

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@310556 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld