MC/COFF: Do not emit forward associative section referenceds.

MSVC link.exe cannot handle associative sections that refer later
sections in the section header. Technically, such COFF object doesn't
violate the Microsoft COFF spec, as the spec doesn't say anything
about that, but still we should avoid doing that to make it compatible
with MS tools.

This patch assigns smaller section numbers to non-associative sections
and larger numbers to associative sections. This should resolve the
compatibility issue.

Differential Revision: https://reviews.llvm.org/D30080

llvm-svn: 295464