[PECOFF] Support COMDAT associative sections.

COFF supports a feature similar to ELF's section groups. This
patch implements it.

In ELF, section groups are identified by their names, and they are
treated somewhat differently from regular symbols. In COFF, the
feature is realized in a more straightforward way. A section can
have an annotation saying "if Nth section is linked, link this
section too."

I added a new reference type, kindAssociate. If a target atom is
coalesced away, the referring atom is removed by Resolver, so that
they are treated as a group.

Differential Revision: http://reviews.llvm.org/D4028

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@211106 91177308-0d34-0410-b5e6-96231b3b80d8