Make DIE.h a public CodeGen header.

dsymutil would like to use all the AsmPrinter/MCStreamer infrastructure
to stream out the DWARF. In order to do so, it will reuse the DIE object
and so this header needs to be public.

The interface exposed here has some corners that cannot be used without a
DwarfDebug object, but clients that want to stream Dwarf can just avoid
these.

Differential Revision: http://reviews.llvm.org/D6695

llvm-svn: 225208