[PATCH] Add DefinedAtom::sectionSize().

[kledzik at apple.com]

It is not ideal to add DefinedAtom::sectionSize() to every platform, when this is only needed in rare cases on one platform.  

A more pay-to-play approach would be to require mergeByLargestSection atoms to have a (group?) reference to a new atom which has a special ContentType and is a placeholder for the section, and its size() method returns the size of the section.  The resolver, when finding a mergeByLargestSection atom looks for the Reference to the magic "section" atom and gets its size for comparison.  The PE/COFF Writer would then also need to ignore atoms of the special contentType atoms.

http://reviews.llvm.org/D4042