AMDGPU: Switch default dwarf version to 2

There were some problems unearthed with version 5,
which I am going to look at.

Differential Revision: https://reviews.llvm.org/D49613



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@337612 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed