[PATCH] [PECOFF] Support yet another new type of weak symbol.

[David Majnemer]

  Note that `llvm::COFF::IMAGE_COMDAT_SELECT_EXACT_MATCH` is still wrongly mapped to `Merge::mergeByContent` because the former operates on a section basis, while the latter operates on an atom basis.

http://llvm-reviews.chandlerc.com/D3011