[PECOFF] Support a new type of weak symbol.

Summary:
COMDAT_SELECT_SAME_SIZE is a COMDAT type that I presume exist only in COFF.
The semantics of the type is that linker should merge such COMDAT sections if
their sizes are the same. Otherwise it's an error.

Reviewers: Bigcheese, shankarke, kledzik

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2996

llvm-svn: 203308