[ELF] Error if the linked-to section of a SHF_LINK_ORDER section is discarded

Summary:
If st_link(A)=B, and A has the SHF_LINK_ORDER flag, we may dereference
a null pointer if B is garbage collected (PR43147):

1. In Wrter.cpp:compareByFilePosition, `aOut->sectionIndex` or `bOut->sectionIndex`
2. In OutputSections::finalize, `d->getParent()->sectionIndex`

Simply error and bail out to avoid null pointer dereferences. ld.bfd has
a similar error:

    sh_link of section `.bar' points to discarded section `.foo0' of `a.o'

ld.bfd is more permissive in that it just checks whether the linked-to
section of the first input section is discarded. This is likely because
it sets sh_link of the output section according to the first input
section.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D67761

llvm-svn: 372400