DebugInfo: Use enum instead of unsigned

This makes debuging DebugInfo generation with LLDB a little more pleasant.

Differential Revision: http://reviews.llvm.org/D3626

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208203 91177308-0d34-0410-b5e6-96231b3b80d8