DebugInfo: Revert change to the return type of createRecordFwdDecl

It still does only return DICompositeType, but I've no need to make that
change right now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188482 91177308-0d34-0410-b5e6-96231b3b80d8