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

[Hans Wennborg via llvm-commits]

zmodem wrote:

> LLVM Buildbot has detected a new failure on builder flang-arm64-windows-msvc

The next build (https://lab.llvm.org/buildbot/#/builders/207/builds/3368) is green, so that was a flake.

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