[ELF] Restore GC handling of LINK_ORDER, C-named sections.

__start_xxx symbol keeps section xxx alive only if it is not
SHF_LINK_ORDER. Such sections can be used for user metadata, when
__start_xxx is used to iterate over section contents at runtime, and
the liveness is determined solely by the linked (associated) section.

This was earlier implemented in r294592, and broken in r296723.

Differential Revision: https://reviews.llvm.org/D30964

llvm-svn: 298154