DebugInfo: Use a 64 bit type for the subrange

While we were encoding 64 bit values (data8) in the subrange itself,
using a 32 bit type for the subrange was still confusing the gdb. Oh,
and make it unsigned too.

As the comment points out, this could be pushed into the frontend so
that it would be 32 or 64 bit as appropriate, etc.

llvm-svn: 205512