[ dwarfdump ] Add symbolic dump of known DWARF attribute values.

Reviewed By: dblaikie

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5187

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217186 91177308-0d34-0410-b5e6-96231b3b80d8