ELF: Extract a non-ELFT base class for VersionNeedSection.

We're going to need a separate VersionNeedSection for each partition, and
the partition data structure won't be templated.

With this the VersionTableSection class no longer needs ELFT, so detemplate it.

Differential Revision: https://reviews.llvm.org/D58808

llvm-svn: 355478