lld-link: Store comdat selection in SectionChunk, reject more invalid associated comdats

I need the comdat selection for PR40094. To keep the patch for that smaller,
I'm adding it here, and as a first application I'm using it to reject
associative comdats referring to earlier associative comdats. Depends on
D56929; together with that all associative comdats referring to other
associative comdats are now rejected.

Differential Revision: https://reviews.llvm.org/D56931

llvm-svn: 352254