[llvm-dwarfdump] - Refactor section name/uniqueness gathering.

As was requested in D36313 thread,

with this patch section names and uniqueness calculated once,
and not every time when a range is dumped.

Differential revision: https://reviews.llvm.org/D36740

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310923 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed