Fix uninitialized variable introduced in r207739.

This was initialized by llvm-mc (calling setDwarfVersion) but other
clients (such as clang, llc, etc) aren't necessarily initializing this
so we were getting garbage DWARF version values in the output.
Initialize it to a reasonable default (the same default used in llvm-mc,
though this is higher than it was (2) previously).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207788 91177308-0d34-0410-b5e6-96231b3b80d8