ELF: Move verneed tracking data structures out of VersionNeedSection.

For partitions I intend to use the same set of version indexes in
each partition for simplicity. Since each partition will need its own
VersionNeedSection this will require moving the verneed tracking out of
VersionNeedSection. The way I've done this is to move most of the tracking
into SharedFile. What will eventually become the per-partition tracking
still lives in VersionNeedSection.

As a bonus the code gets a little simpler and more consistent with how we
handle verdef.

Differential Revision: https://reviews.llvm.org/D60307

llvm-svn: 357926