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

COMDAT_SELECT_LARGEST is a COMDAT type that make linker to choose the largest
definition from among all of the definition of a symbol. If the size is the
same, the choice is arbitrary.

Reviewers: Bigcheese, shankarke, kledzik

CC: llvm-commits

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