Delete the copy constructor for llvm::yaml::Node

The nodes keep a reference back to the original document, but the
document is streamed, not read all into memory at once, and the
position is part of the state. If nodes are ever copied, the document
position can end up being advanced more than once.

This did not reveal any problems in LLVM or Clang but caught a handful
over in Swift!

llvm-svn: 328345