Fix asm printing of associated sections.

Fix asm printing of associated sections.

Make MCSectionELF::AssociatedSection be a link to a symbol, because
that's how it works in the assembly, and use it in the asm printer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297769 91177308-0d34-0410-b5e6-96231b3b80d8