[PATCH] Add a fallback mechanism for undefined atom.

[Rui Ueyama]

  So as I wrote a mergeable symbol with internal linkage is a weird concept, as it's obscure whether it should really be merged with symbols in other object file (because it's mergeable) or shouldn't (because it's private). Also, isAlias is not really implemented at all except some stub function to return boolean value, so I don't have a scaffold to connect this "weak external" thing to, to try to see how it'll work with that. Implementing the alias thing to support this COFF feature sounds like a bit too much to me.

http://llvm-reviews.chandlerc.com/D1550