DebugInfo: Don't create duplicate forward declaration metadata unnecessarily.

No functionality change, at best a slight (questionable) optimization,
but necessary for correctness of future work.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188474 91177308-0d34-0410-b5e6-96231b3b80d8
DebugInfo: remove unnecessary type registration

This happens in the caller a few frames up anyway.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188475 91177308-0d34-0410-b5e6-96231b3b80d8