[llvm] [MsDemangle] Use LLVM style RTTI for AST nodes (PR #143410)

[Hans Wennborg via llvm-commits]

zmodem wrote:

> One thing that's missing without RTTI is the ability to determine if a node is a symbol, identifier, or type (or one would need to check for every kind).

Do you have an example in mind, where this added RTTI functionality would make the code simpler?

https://github.com/llvm/llvm-project/pull/143410