ELF: Don't merge SHF_LINK_ORDER sections for different output sections in relocatable links.

Merging SHF_LINK_ORDER sections can affect semantics if the sh_link
fields point to different sections.

Specifically, for SHF_LINK_ORDER sections, the sh_link field acts as a reverse
dependency from the linked section, causing the SHF_LINK_ORDER section to
be included if the linked section is included. Merging sections with different
sh_link fields will cause the entire contents of the SHF_LINK_ORDER section
to be associated with a single (arbitrarily chosen) output section, whereas the
correct semantics are for the individual pieces of the SHF_LINK_ORDER section
to be associated with their linked output sections. As a result we can end up
incorrectly dropping SHF_LINK_ORDER section contents or including the wrong
section contents, depending on which linked sections were chosen.

Differential Revision: https://reviews.llvm.org/D68094

llvm-svn: 373255