[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.

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

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@204172 91177308-0d34-0410-b5e6-96231b3b80d8