DebugInfo: Flag type unit references as declarations

This ensures DWARF consumers don't confuse these references for
definitions. I'd argue it might be nice to improve debuggers so we don't
need this, but it's just one field in an abbreviation anyway - so it
doesn't seem worth the fight.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200569 91177308-0d34-0410-b5e6-96231b3b80d8