MC: Fix associative sections on COFF

COFF sections in MC were represented by a tuple of section-name and
COMDAT-name.  This is not sufficient to represent a .text section
associated with another .text section; we need a way to distinguish
between the key section and the one marked associative.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211913 91177308-0d34-0410-b5e6-96231b3b80d8