[ELF] Support input section description .gnu.version* in /DISCARD/

Linux powerpc discards `*(.gnu.version*)` (arch/powerpc/kernel/vmlinux.lds.S)
to suppress --orphan-handling=warn warnings in the -pie output `.tmp_vmlinux1`

The support is simple. Just add isLive() to:

1) Fix an assertion in SectionBase::getPartition() called by VersionTableSection::isNeeded().
2) Suppress DT_VERSYM, DT_VERDEF, DT_VERNEED and DT_VERNEEDNUM, if the relevant section is discarded.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D71819