[PATCH] Add DefinedAtom::sectionSize().

[Simon Atanasyan]

I like this idea because now I try to figure out how to propagate the `--as-needed` from the `ELFFileNode::parse()` to the point where we create the `DT_NEEDED` dynamic tag. Your idea might be a solution to this problem too.

http://reviews.llvm.org/D4042