[ELF] - Do not crash when do --gc-sections for non-allocatable metadata sections.

Currently, LLD marks all non-allocatable sections except SHF_REL[A] as Live
when doing GC.

This can be a reason of the crash when SHF_LINK_ORDER sections
are involved, because their parents can be dead.

We should do GC for them correctly. The patch implements it.

Differential revision: https://reviews.llvm.org/D46880

llvm-svn: 332589