Fix a bug in ICF involving COFF associative sections.

Associative sections are sections that need to be linked if their associated
sections are linked. Associative sections are used to append auxiliary data
such as debug info.

Previously, we compared all associative sections when comparing two comdat
sections. Because usually assocative sections are not mergeable sections,
we missed a lot of mergeable sections. MSVC linker doesn't seem to check
the identity of associative sections.

This patch makes LLD to ignore associative sections when doing ICF.

llvm-svn: 288483