[CFGPrinter] Add node id formater (#164623)
This PR is part of the LLVM IR LSP server project
([RFC](https://discourse.llvm.org/t/rfc-ir-visualization-with-vs-code-extension-using-an-lsp-server/87773))
Sometimes it is nice to be able to specify IDs of nodes in the printed
CFG. For better manipulation of the outputed CFG.
In our case we will use it for navigation between IR and CFG views.
This adds an argument to DOTFuncInfo - a function that takes a
BasicBlock and returns a node ID, to be printed in the result dot.
