[PATCH] Add DefinedAtom::sectionSize().

[Rui Ueyama]

DefinedAtom::sectionSize() is basically free if your atom does not have mergeByLargestSection attribute. It's a virtual function so it'll use one more slot for vftable, but that's it. There's no extra cost for each atomwhich does not use mergeByLargestSection. Making a virtual group atom only for size would work, but it's too complicated.

http://reviews.llvm.org/D4042