[AMDGPU] Set default dwarf version to 2

Differential Revision: http://reviews.llvm.org/D20640

llvm-svn: 271347
diff --git a/clang/lib/Driver/ToolChains.h b/clang/lib/Driver/ToolChains.h
index 244c22a..030943d 100644
--- a/clang/lib/Driver/ToolChains.h
+++ b/clang/lib/Driver/ToolChains.h
@@ -943,6 +943,7 @@
 public:
   AMDGPUToolChain(const Driver &D, const llvm::Triple &Triple,
             const llvm::opt::ArgList &Args);
+  unsigned GetDefaultDwarfVersion() const override { return 2; }
   bool IsIntegratedAssemblerDefault() const override { return true; }
 };