[ELF] Move SHF_LINK_ORDER till OutputSection addresses are known

Sections with the SHF_LINK_ORDER flag must be ordered in the same relative
order as the Sections they have a link to. When using a linker script an
arbitrary expression may be used for the virtual address of the
OutputSection. In some cases the virtual address does not monotonically
increase as the OutputSection index increases, so if we base the ordering
of the SHF_LINK_ORDER sections on the index then we can get the order
wrong. We fix this by moving SHF_LINK_ORDER resolution till after we have
created OutputSection virtual addresses.

Differential Revision: https://reviews.llvm.org/D79286