[llvm-readobj] - Teach llvm-readobj to print dependencies of SHT_GNU_verdef and refac...

[llvm-readobj] - Teach llvm-readobj to print dependencies of SHT_GNU_verdef and refactor dumping method.

This patch changes single method of llvm-readobj.
It teaches SHT_GNU_verdef dumper to print version dependencies,
also it removes few fields from output that can be dumped with other keys
and slightly refactors code.
Testcase was also modified to match the changes.
Change is required for testcases of upcoming lld patches.

Differential revision: http://reviews.llvm.org/D21552

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273417 91177308-0d34-0410-b5e6-96231b3b80d8